CS6910: Fundamentals of Deep Learning
Lecture 7: Greedy Layerwise Pre-training, Better activation functions, Better weight initialization, Batch Normalization
Mitesh M. Khapra, Arun Prakash A
AI4Bharat, Department of Computer Science and Engineering, IIT Madras
Module 7.1 : A quick recap of training deep neural networks
Mitesh M. Khapra
AI4Bharat, Department of Computer Science and Engineering, IIT Madras
We already saw how to train this network
\nabla w_t = \frac{\partial \mathscr{L(w)}}{\partial w_t}
What about a wider network with more inputs:
where, \( \nabla w_i\) = \((f(x) − y) ∗ f(x) ∗ (1 − f(x)) ∗ \textcolor{red}{x_i}\)
\(w_1\) = \(w_1 - \eta \nabla w_1\)
y
x
\sigma
w
\(w_{t+1} = w_t − \eta \nabla w_t\) where,
=(f(x) − y) ∗ f(x) ∗ (1 − f(x)) ∗ x
y
\sigma
x_1
x_3
x_2
w_1
w_3
w_2
\(w_2\) = \(w_2 - \eta \nabla w_2\)
\(w_3\) = \(w_3 - \eta \nabla w_3\)
What if we have a deeper network ?
We can now calculate \(\nabla w_1\) using chain rule:
\frac{\partial \mathscr{L(w)}}{\partial w_1}=\frac{\partial \mathscr{L(w)}}{\partial y}.\frac{\partial y}{\partial a_3}.\frac{\partial a_3}{\partial h_2}.\frac{\partial h_2}{\partial a_2}.\frac{\partial a_2}{\partial h_1}.\frac{\partial h_1}{\partial a_1}.\frac{\partial a_1}{\partial w_1}
\(a_i = w_ih_{i−1}; h_i = σ(a_i)\)
\(a_1 = w_1 ∗ x = w_1 ∗ h_0\)
∗ ............... ∗ \(h_0\)
=\frac{\partial \mathscr{L(w)}}{\partial y}
y
h_2
a_3
w_3
a_2
w_2
h_1
a_1
w_1
\sigma
\sigma
\sigma
\(x=h_0\)
In general,
\( \)
\nabla w_i=\frac{\partial \mathscr{L(w)}}{\partial y}
∗ ............... ∗ \(h_i-1\)
Notice that \( \nabla w_i\) is proportional to the corresponding input \(h_i−1\)
(we will use this fact later)
What happens if we have a network which is deep and wide?
How do you calculate \( \nabla w_2\)=?
It will be given by chain rule applied across multiple paths
\sigma
w_1
w_2
w_3
x_1
x_2
x_3
y
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
What happens if we have a network which is deep and wide?
How do you calculate \( \nabla w_2\)=?
It will be given by chain rule applied across multiple paths
\sigma
w_1
w_2
w_3
x_1
x_2
x_3
y
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
What happens if we have a network which is deep and wide?
How do you calculate \( \nabla w_2\)=?
It will be given by chain rule applied across multiple paths
\sigma
w_1
w_2
w_3
x_1
x_2
x_3
y
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
What happens if we have a network which is deep and wide?
How do you calculate \( \nabla w_2\)=?
It will be given by chain rule applied across multiple paths
\sigma
w_1
w_2
w_3
x_1
x_2
x_3
y
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
What happens if we have a network which is deep and wide?
How do you calculate \( \nabla w_2\)=?
It will be given by chain rule applied across multiple paths (We saw this in detail when we studied back propagation)
\sigma
w_1
w_2
w_3
x_1
x_2
x_3
y
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
Training Neural Networks is a Game of Gradients (played using any of the existing gradient based approaches that we discussed)
Things to remember
The gradient tells us the responsibility of a parameter towards the loss
The gradient w.r.t. a parameter is proportional to the input to the parameters (recall the “..... ∗ x” term or the “.... ∗ \(h_{i-1}\)” term in the formula for \( \nabla w_i\))
Backpropagation was made popular by Rumelhart et.al in 1986
However when used for really deep networks it was not very successful
In fact, till 2006 it was very hard to train very deep networks
Typically, even after a large number of epochs the training did not converge
\sigma
w_2
w_3
x_1
x_2
x_3
y
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
\sigma
Module 7.2 : Unsupervised pre-training
Mitesh M. Khapra
AI4Bharat, Department of Computer Science and Engineering, IIT Madras
What has changed now? How did Deep Learning become so popular despite this problem with training large networks?
What has changed now? How did Deep Learning become so popular despite this problem with training large networks?
Well, until 2006 it wasn’t so popular
What has changed now? How did Deep Learning become so popular despite this problem with training large networks?
Well, until 2006 it wasn’t so popular
The field got revived after the seminal work of Hinton and Salakhutdinov in 2006
Let’s look at the idea of unsupervised pre-training introduced in this paper ...
Let’s look at the idea of unsupervised pre-training introduced in this paper ...
(note that in this paper they introduced the idea in the context of RBMs but we will discuss it in the context of Autoencoders)
Consider the deep neural network shown in this figure
Let us focus on the first two layers of the network \(x\) and \(h_1\)
We will first train the weights between these two layers using an unsupervised objective
Note that we are trying to reconstruct the input (\(x\)) from the hidden representation ( \(h_1\))
We refer to this as an unsupervised objective because it does not involve the output label (\(y\)) and only uses the input data (\(x\))
min \frac{1}{m} \sum \limits_{i=1}^m \sum \limits_{j=1}^n (\hat{x}_{ij}-x_{ij})^2
Reconstruct \(x\)
At the end of this step, the weights in layer 1 are trained such that \(h_1\) captures an abstract representation of the input \(x\)
We now fix the weights in layer 1 and repeat the same process with layer 2
At the end of this step, the weights in layer 2 are trained such that captures an abstract representation of \(h_1\)
We continue this process till the last hidden layer (i.e., the layer before the output layer) so that each successive layer captures an abstract representation of the previous layer
min \frac{1}{m} \sum \limits_{i=1}^m \sum \limits_{j=1}^n (\hat{h}_{1_{ij}}-h_{1_{ij}})^2
x
h_1
h_2
\hat{h_1}
After this layerwise pre-training, we add the output layer and train the whole network using the task specific objective
x_1
x_2
x_3
\argmin \frac{1}{m} \sum \limits_{i=1}^m (\hat{y}_i-f(x_i))^2
Note that, in effect we have initialized the weights of the network using the greedy unsupervised objective and are now fine tuning these weights using the supervised objective
Why does this work better?
1 The difficulty of training deep architectures and effect of unsupervised pre-training - Erhan et al,2009
2 Exploring Strategies for Training Deep Neural Networks, Larocelle et al,2009
Is it because of better optimization?
Is it because of better regularization?
Let’s see what these two questions mean and try to answer them based on some (among many) existing studies [1,2]
Why does this work better?
Is it because of better optimization?
Is it because of better regularization?
What is the optimization problem that we are trying to solve?
\text{minimize}\ \mathscr L(\theta)=\frac{1}{m} \displaystyle \sum_{i=1}^m (y_i - f(x_i))^2
Is it the case that in the absence of unsupervised pre-training we are not able to drive (\(\mathscr{L}(\theta)\) ) to 0 even for the training data (hence poor optimization) ?
Let us see this in more detail ...
The error surface of the supervised objective of a Deep Neural Network is highly non-convex
With many hills and plateaus and valleys
Given that large capacity of DNNs it is still easy to land in one of these 0 error regions
Indeed Larochelle et.al.1 show that if the last layer has large capacity then \(\mathscr{L}(\theta)\) goes to 0 even without pretraining
However, if the capacity of the network is small, unsupervised pretraining helps
Why does this work better?
Is it because of better optimization?
Is it because of better regularization?
What does regularization do? It constrains the weights to certain regions of the parameter space
L-1 regularization: constrains most weights to be 0
L-2 regularization: prevents most weights from taking large values
L_1
L_2
w_1
w_2
w_1
w_2
w^*
w^*
Indeed, pre-training constrains the weights to lie in only certain regions of the parameter space
Specifically, it constrains the weights to lie in regions where the characteristics of the data are captured well (as governed by the unsupervised objective)
\Omega(\theta)=\frac{1}{m} \displaystyle \sum_{i=1}^m \sum_{j=1}^n (x_{ij} - \hat{x}_{ij})^2
Unsupervised objective:
We can think of this unsupervised objective as an additional constraint on the optimization problem
Supervised objective:
\mathscr L(\theta)=\frac{1}{m} \displaystyle \sum_{i=1}^m (y_i - f(x_i))^2
This unsupervised objective ensures that that the learning is not greedy w.r.t. the supervised objective (and also satisfies the unsupervised objective)
Some other experiments have also shown that pre-training is more robust to random initializations
1The difficulty of training deep architectures and effect of unsupervised pre-training - Erhan et al,2009
One accepted hypothesis is that pretraining leads to better weight initializations (so that the layers capture the internal characteristics of the data)
So what has happened since 2006-2009?
Deep Learning has evolved
Better optimization algorithms
Better regularization methods
Better Activation functions
Better weight initialization strategies
Module 7.3 : Better activation functions
Mitesh M. Khapra
AI4Bharat, Department of Computer Science and Engineering, IIT Madras
Deep Learning has evolved
Better optimization algorithms
Better regularization methods
Better Activation functions
Better weight initialization strategies
Before we look at activation functions, let’s try to answer the following question: “What makes Deep Neural Networks powerful ?”
Consider this deep neural network
Imagine if we replace the sigmoid in each layer by a simple linear transformation
\(y = (w_4 ∗ (w_3 ∗ (w_2 ∗ (w_1*x)))) \)
Then we will just learn \(y\) as a linear transformation of \(x\)
In other words we will be constrained to learning linear decision boundaries
We cannot learn arbitrary decision boundaries
y
h_2
a_3
w_3
a_2
w_2
h_1
a_1
w_1
\sigma
\sigma
\sigma
\(h_0=x\)
In particular, a deep linear neural network cannot learn such boundaries
But a deep non linear neural network can indeed learn such boundaries (recall Universal Approximation Theorem)
Now let’s look at some non-linear activation functions that are typically used in deep neural networks (Much of this material is taken from Andrej Karpathy’s lecture notes 1 )
\sigma(x)=\frac{1}{1+e^{-x}}
As is obvious, the sigmoid function compresses all its inputs to the range [0,1]
Since we are always interested in gradients, let us find the gradient of this function
\frac{\partial \mathcal{\sigma(x)}}{\partial x}=\sigma(x)(1-\sigma(x))
Let us see what happens if we use sigmoid in a deep network
Sigmoid
x
y
While calculating \(\nabla w_2\) at some point in the chain rule we will encounter
\frac{\partial \mathcal{h3}}{\partial a_3}=\frac{\partial \mathcal{\sigma(a_3)}}{\partial a_3}=\sigma(a_3)(1-\sigma(a_3))
What is the consequence of this ?
To answer this question let us first understand the concept of saturated neuron ?
a_4
h_2
a_1
\sigma
\sigma
\sigma
\(h_0=x\)
h_3
h_4
a_3
h_1
a_2
\sigma
\sigma
\sigma
a_3=w_3h_2
h_3=\sigma(a_3)
A sigmoid neuron is said to have saturated when \(\sigma(x)=0\) or \(\sigma(x)=1\)
What would the gradient be at saturation?
Well it would be 0 (you can see it from the plot or from the formula that we derived)
Saturated neurons thus cause the gradient to vanish.
x
y
Saturated neurons thus cause the gradient to vanish.
But why would the neurons saturate ?
w_1
w_2
w_3
w_4
x
y
Consider what would happen if we use sigmoid neurons and initialize the weights to very high values ?
The neurons will saturate very quickly
The gradients will vanish and the training will stall (more on this later)
\sigma(\sum \limits_{i=1}^nw_ix_i)
Saturated neurons thus cause the gradient to vanish.
Sigmoids are not zero centered
Why is this a problem??
Consider the gradient w.r.t. \(w_1\) and \(w_2\)
\nabla w_1=\textcolor{red}{\frac{\partial \mathscr{L(w)}}{\partial y} \frac{\partial \mathcal{y}}{\partial h_3} \frac{\partial \mathcal{h_3}}{\partial a_3}
} \textcolor{blue}{h_{21}}
\nabla w_2=\textcolor{red}{\frac{\partial \mathscr{L(w)}}{\partial y} \frac{\partial \mathcal{y}}{\partial h_3} \frac{\partial \mathcal{h_3}}{\partial a_3}} \textcolor{blue}{h_{22}}
Note that \( \textcolor{blue}{h_{21}}\) and \( \textcolor{blue}{h_{22}}\) are between [0, 1] (i.e., they are both positive)
So if the first common term (in red) is positive (negative) then both \( \nabla w_1\) and \( \nabla w_2\) are positive (negative)
Essentially, either all the gradients at a layer are positive or all the gradients at a layer are negative
y
h_2
w_2
w_1
h_1
\(h_0=x\)
a_3=w_1h_{21}+w_2h_{22}
Saturated neurons thus cause the gradient to vanish.
Sigmoids are not zero centered
This restricts the possible update directions
\nabla w_1
\nabla w_2
Saturated neurons thus cause the gradient to vanish.
Sigmoids are not zero centered
This restricts the possible update directions
\nabla w_1
\nabla w_2
(Not possible)
(Not possible)
Quadrant in which
all gradients are
+ve
(Allowed)
Quadrant in which
all gradients are
-ve
(Allowed)
Saturated neurons thus cause the gradient to vanish.
Sigmoids are not zero centered
This restricts the possible update directions
\nabla w_1
\nabla w_2
(Not possible)
(Not possible)
Quadrant in which
all gradients are
+ve
(Allowed)
Quadrant in which
all gradients are
-ve
(Allowed)
Now imagine:
this is the
optimal w
Saturated neurons thus cause the gradient to vanish.
Sigmoids are not zero centered
This restricts the possible update directions
\nabla w_1
\nabla w_2
Saturated neurons thus cause the gradient to vanish.
Sigmoids are not zero centered
This restricts the possible update directions
\nabla w_1
\nabla w_2
initial position
only way to reach it is
by taking a zigzag path
starting from this
Saturated neurons thus cause the gradient to vanish.
Sigmoids are not zero centered
This restricts the possible update directions
\nabla w_1
\nabla w_2
initial position
only way to reach it is
by taking a zigzag path
starting from this
And lastly, sigmoids are computationally expensive (because of exp (x))
Compresses all its inputs to the range [-1,1]
Zero centered
What is the derivative of this function?
\frac{\partial \mathcal{tanh(x)}}{\partial x}=(1-tanh^2(x))
The gradient still vanishes at saturation
Also computationally expensive
\(tanh(x)\)
Is this a non-linear function?
\(f(x) = max(0, x) \)
ReLU (Rectified Linear Unit)
x
f(x)
Indeed it is!
\(f(x) = max(0, x ) − max(0, x − 6)\)
Is this a non-linear function?
Indeed it is!
In fact we can combine two ReLU units to recover a piecewise linear approximation of the scaled sigmoid function
It is also called ReLU6 (6 denotes the maximum value of the ReLU)
Advantages of ReLU
Does not saturate in the positive region
\(f(x)= max(0, x)\)
\(^1\)ImageNet Classification with Deep Convolutional Neural Networks- Alex Krizhevsky Ilya Sutskever, Geoffrey E. Hinton, 2012
ReLU (Rectified Linear Unit)
Computationally efficient
In practice converges much faster than \(sigmoid/tanh^1\)
In practice there is a caveat
Let’s see what is the derivative of ReLU(x)
\frac{\partial {ReLU(x)}}{\partial x}
\(=1 if x > 0\)
\(=0 if x < 0\)
Now consider the given network
What would happen if at some point a large gradient causes the bias \(b\) to be updated to a large negative value?
y
w_3
w_1
a_2
h_1
a_1
w_2
x_1
x_2
b
1
\(w_1x_1 + w_2x_2 + b < 0 [if b << 0]\)
The neuron would output 0 [dead neuron]
Not only would the output be 0 but during backpropagation even the gradient \( \frac{\partial h_1}{\partial a_1} \)
would be zero
The weights \(w_1\), \(w_2\) and b will not get updated [∵ there will be a zero term in the chain rule]
\nabla w_1=\frac{\partial \mathscr{L(\theta)}}{\partial y} \frac{\partial \mathcal{y}}{\partial a_2} \frac{\partial \mathcal{a_2}}{\partial h_1} \frac{\partial \mathcal{h_1}}{\partial a_1} \frac{\partial \mathcal{a_1}}{\partial w_1}
The neuron will now stay dead forever!!
y
w_3
w_1
a_2
h_1
a_1
w_2
x_1
x_2
b
1
In practice a large fraction of ReLU units can die if the learning rate is set too high
It is advised to initialize the bias to a positive value (0.01)
Use other variants of ReLU (as we will soon see)
y
w_3
w_1
a_2
h_1
a_1
w_2
x_1
x_2
b
1
No saturation
\(f(x) = max(0.1x,x) \)
Leaky ReLU
Will not die (0.1x ensures that at least a small gradient will flow through)
Computationally efficient
Close to zero centered ouputs
\( f(x) = max(αx, x)\)
α is a parameter of the model
α will get updated during backpropagation
Parametric ReLU
All benefits of ReLU
Exponential Linear Unit (ELU)
\(f(x) = x if x > 0\)
\(= ae^x − 1 if x ≤ 0\)
\(ae^x − 1\) ensures that at least a small gradient will flow through
Close to zero centered outputs
Expensive (requires computation of exp(x))
Generalizes ReLU and Leaky ReLU
Maxout Neuron: \(max(w_1x+b_1,\cdots,w_nx+b_n\))
\(max(0.5 x , -0.5 x)\)
No saturation ! No death!
For k affine transformation in a neuron, k times increase in number of parameters
\(max(0 x + 0, w_ 2^T x + b_2)\)
Two MaxOut neurons, with sufficient number of affine transformations, act as a universal approximator!
\(max(0.5 x , -0.5 x,x,-x-0.2)\)
Let's draw some similarities between hard threshold activation function and ReLU activation function
f(x)=
\begin{cases}
1 ,& \text{if } x > 0\\
0, & x \leq 0
\end{cases}
GELU (Gaussian Error Linear Unit)
GELU (Gaussian Error Linear Unit)
Let's draw some similarities between hard threshold activation function and ReLU activation function
Both activation functions output the value based on the sign of the input
f(x)=
\begin{cases}
1 ,& \text{if } x > 0\\
0, & x \leq 0
\end{cases}
f(x)=
\begin{cases}
1 \cdot x ,& \text{if } x > 0\\
0, & x \leq 0
\end{cases}
Let's rewrite the ReLU activation a bit as follows
f(x)=
\begin{cases}
1 \cdot x ,& \text{if } x > 0\\
0 \cdot x, & x \leq 0
\end{cases}
The ReLU function multiplies the input by 1 or 0.
The Dropout also does the same.
But with one difference
It does it stochastically as follows (with a slight abuse of notations)
Therefore, it make sense to combine these properties into the activation function.
\mu(x)=
\begin{cases}
1 \cdot x ,& p\\
0 \cdot x, & 1-p
\end{cases}
f(x)=
m \cdot x
m \sim Bernoulli(\Phi(x))
where,
Now, the multiplication factor \(m\) is random and also function of input \(\Phi(x)\)
The range of \(\Phi(x)\) has to be within \(0 \leq \Phi(x) \leq 1\)
\(\Phi(x)\) can be logistic, however, the more natural choice is cumulative distribution of standard normal distribution
The expected value of \(f(x) = m.x\) is
E(f(x))=
E(m \cdot x )
=\Phi(x)\cdot1\cdot x +(1-\Phi(x)) \cdot 0 \cdot x
=\Phi(x)x
=P(X \leq x)x
GELU(x)
Often, CDF of Gaussian (\(\mu=0,\sigma=1\)) is computed with the error function.The approximation of error function is given by
\approx0.5x(1+tanh[\frac{\sqrt{2}} {\sqrt{\pi}}(x+0.044715x^3)])
\approx x \sigma(1.702x)
SELU (Scaled Exponential Linear Unit)
It is helpful to center the output activations from each layer to zero mean and unit variance
We can do this explicitly using normalization techniques (we will see soon)
What if we do this with the activation function itself (called self normalizing)
SELU does it!
f(x)= \lambda
\begin{cases}
x \quad \quad \quad if \quad x>0 \\
\alpha e^x-\alpha \quad if \quad x \leq 0
\end{cases}
\lambda \approx 1.0507009
\lambda \approx 1.673263
Automatic Search for Activation functions
\frac{1}{1+e^{(-x)}}
max(x)
tanh(x)
x\sigma(1.702x)
max(\alpha x,x)
Assume there exists a search space for activation function (scalar functions)
All such scalar functions are composed of two operations
Unary: (\(|x|,-x,exp(-x),\sqrt(x)\))
Binary: (\(max(x_1,x_2), x_1 \cdot \sigma(x_2)\))
Then, it is possible to search the space to find the best activation function which improves the accuracy of the existing models
Automatic Search for Activation functions
Figure on the right shows top four activation functions found by the search method.
According to the search method, the best activation takes the following form
f(x)=x \sigma(\beta x)
It is named as SWISH. Here, \(\beta\) can be a constant like in GELU \(\beta=1.702\) or a learnable parameter
SILU (Sigmoid-weighted Linear Unit)
We call \(x \sigma(1.702x)\) as GELU
We call \(x \sigma(\beta x)\) as SWISH
What should we call \(x \sigma(x)\) ?
Interestingly, it was proposed in the Reinforcement learning paradigm
Things to Remember
Sigmoids are bad
ReLU is more or less the standard unit for Convolutional Neural Networks
Can explore Leaky ReLU/Maxout/ELU
tanh sigmoids are still used in LSTMs/RNNs (we will see more on this later)
GELU is most commonly used in Transformer based architectures like BERT, GPT
Module 7.4 : Better initialization strategies
Mitesh M. Khapra
AI4Bharat, Department of Computer Science and Engineering, IIT Madras
Deep Learning has evolved
Better optimization algorithms
Better regularization methods
Better activation functions
Better weight initialization strategies
For learning to happen, the Gradients must flow from the Output layer to the Input layer.
One of the factors that affects this flow is the saturated neurons in a network.
\nabla w=(f(x)-y)\sigma(x)(1-\sigma(x) *input
How do we prevent neurons from getting saturated?
However, initializing the parameters is a function of number of inputs \(fan_{in}\), number of outputs \(fan_{out}\),type of activation function and type of architecture.
By carefully initializing the parameters of the network.
What happens if we initialize all weights to 0?
\(a_{11} = w_{11}x_1 + w_{12}x_2 \)
\(a_{12} = w_{21}x_1 + w_{22}x_2 \)
\(a_{11} =a_{12} = 0 \)
\(h_{11} =h_{12} \)
All neurons in layer 1 will get the same activation
\therefore
\therefore
y
h_{21}
a_{21}
h_{11}
x_1
x_2
a_{11}
h_{13}
a_{13}
h_{12}
a_{12}
\sigma
\sigma
\sigma
\sigma
Now what will happen during back propagation?
\(a_{11} = w_{11}x_1 + w_{12}x_2 \)
\(a_{12} = w_{21}x_1 + w_{22}x_2 \)
\nabla w_{11}=\frac{\partial \mathscr{L(w)}}{\partial y}.\frac{\partial \mathscr{y}}{\partial h_{11}}.\frac{\partial \mathscr{h_{11}}}{\partial a_{11}}.x_1
\nabla w_{21}=\frac{\partial \mathscr{L(w)}}{\partial y}.\frac{\partial \mathscr{y}}{\partial h_{12}}.\frac{\partial \mathscr{h_{11}}}{\partial a_{12}}.x_1
\(but h_{11} =h_{12} \)
\(and a_{11} =a_{12} \)
\( \nabla w_{11} =\nabla w_{21} \)
\(a_{11} =a_{12} = 0 \)
\(h_{11} =h_{12} \)
\therefore
\therefore
y
h_{21}
a_{21}
h_{11}
x_1
x_2
a_{11}
h_{13}
a_{13}
h_{12}
a_{12}
\sigma
\sigma
\sigma
\sigma
\(a_{11} = w_{11}x_1 + w_{12}x_2 \)
\(a_{12} = w_{21}x_1 + w_{22}x_2 \)
Hence both the weights will get the same update and remain equal
Infact this symmetry will never break during training
The same is true for \(w_{12}\) and \(w_{22}\)
And for all weights in layer 2 (infact, work out the math and convince yourself that all the weights in this layer will remain equal )
This is known as the symmetry breaking problem
This will happen if all the weights in a network are initialized to the same value
\(a_{11} =a_{12} = 0 \)
\(h_{11} =h_{12} \)
\therefore
\therefore
y
h_{21}
a_{21}
h_{11}
x_1
x_2
a_{11}
h_{13}
a_{13}
h_{12}
a_{12}
\sigma
\sigma
\sigma
\sigma
We will now consider a feedforward network with:
input: 1000 points, each ∈ \(R^{500}\)
input data is drawn from unit Gaussian
the network has 5 layers
each layer has 500 neurons
we will run forward propagation on this network with different weight initializations
num_layers = 5
D = np.random.randn(1000,500)
for i in range(num_layers):
x = D if i==0 else h_cache[i-1]
W = 0.01*np.random.randn(500,500)
a = np.dot(x,W)
h = sigmoid(a)num_layers = 5
D = np.random.randn(1000,500)
for i in range(num_layers):
x = D if i==0 else h_cache[i-1]
W = 0.01*np.random.randn(500,500)
a = np.dot(x,W)
h = sigmoid(a)Let’s try to initialize the weights to small random numbers
We will see what happens to the activation across different layers with sigmoid activation functions
W = 0.01*np.random.randn(500,500) 
Let’s try to initialize the weights to small random numbers
We will see what happens to the activation across different layers with tanh activation functions
W = 0.01*np.random.randn(500,500) 
What will happen during back propagation?
Recall that \( \nabla w_1\) is proportional to the activation passing through it
If all the activations in a layer are very close to 0, what will happen to the gradient of the weights connecting this layer to the next layer?
They will all be close to 0 (vanishing gradient problem)
Now, let's experiment with the real world data and see what happens
Data: MNIST, \(x \in \mathbb{R}^{784}\)
Model: FFNN with three hidden layers
h_1, h_2,h_3\in \mathbb{R}^{500}
Loss: Cross entropy
Optimizer: Mini-Batch GD
\eta=0.5
Batch size: 256
Activation: tanh
h_1
h_2
h_3
\mathscr{L}(\theta)
W = 0.01*np.random.randn(500,500) Observe that for the first 35 iterations, the loss (almost) remains constant
Let us try to initialize the weights to large random numbers
W = np.random.randn(500,500) \sigma()
Let us try to initialize the weights to large random numbers
W = np.random.randn(500,500) \tanh()
Most activations have saturated
What happens to the gradients at saturation?
They will all be close to 0 (vanishing gradient problem)
Now, let's experiment with the real world data and see what happens
Data: MNIST, \(x \in \mathbb{R}^{784}\)
Model: FFNN with three hidden layers
h_1, h_2,h_3\in \mathbb{R}^{500}
Loss: Cross entropy
Optimizer: Mini-Batch GD
\eta=0.5
Batch size: 256
Activation: tanh
h_1
h_2
h_3
\mathscr{L}(\theta)
W = np.random.randn(500,500) Let us try to arrive at a more principled way of initializing weights
s_{11}= \displaystyle \sum_{i=1}^n w_{1i}x_i
\text{Var}(s_{11})=\text{Var}\displaystyle \sum_{i=1}^n w_{1i}x_i=\displaystyle \sum_{i=1}^n\text{Var}( w_{1i}x_i)
+ (E[x_i])^2 Var(w_{1i})+ Var(x_i)Var(w_{1i})]
= \displaystyle \sum_{i=1}^n[(E[w_{1i}])^2 Var(x_i)
[Assuming 0 Mean inputs and weights]
= \displaystyle \sum_{i=1}^n Var(x_i) Var(w_{1i})
[Assuming \(V ar(xi) = V ar(x)∀i ]\)
[Assuming
\(V ar(w_{1i}) = V ar(w)∀i]\)
=(n Var(w))(Var(x))
s_{11}
s_{1n}
x_1
x_2
x_3
x_n
In general,
Var(S_{1i}) = (n Var(w))(Var(x))
What would happen if \(nVar(w) \gg 1\)
?
s_{11}
s_{1n}
x_1
x_2
x_3
x_n
The variance of \(S_{1i}\) will be large
What would happen if \(nV ar(w) → 0?\)
The variance of \(S_{1i}\) will be small
Let us see what happens if we add one more layer
Using the same procedure as above we will arrive at
Var(s_{21})=\displaystyle \sum_{i=1}^n Var(s_{1i}) Var(w_{2i})
=n Var(s_{1i})Var(w_2)
Var(S_{i1}) = nVar(w_1)Var(x)
Var(s_{21}) \propto [n Var(w_2)][nVar(w_1)]Var(x)
\propto [nVar(w)]^2Var(x)
Assuming weights across all layers have the same variance
s_{21}
s_{1n}
x_1
x_2
x_3
x_n
s_{11}
In general,
Var(s_{ki}) = [nV ar(w)]^kVar(x)
To ensure that variance in the output of any layer does not blow up or shrink we want:
nVar(w) = 1
If we draw the weights from a unit Gaussianand scale them by then, we have ,\(\frac{1}{\sqrt{n}} \)
nVar(w)=nVar(\frac{z}{\sqrt{\smash[b]{n}}} )
=n * \frac{1}{n} Var(z)= 1
Var(az) = a^2 (Var(z))
← (UnitGaussian)
Let’s see what happens if we use this initialization
W = np.random.randn(fan_in,fan_out)/np.sqrt(fan_in)
\tanh(h_i)
Let’s see what happens if we use this initialization
W = np.random.randn(fan_in,fan_out)/np.sqrt(fan_in)\sigma(h_i)
Now, let's experiment with the real world data and see what happens
Data: MNIST, \(x \in \mathbb{R}^{784}\)
Model: FFNN with three hidden layers
h_1, h_2,h_3\in \mathbb{R}^{500}
Loss: Cross entropy
Optimizer: Mini-Batch GD
\eta=0.5
Batch size: 256
Activation: tanh
h_1
h_2
h_3
\mathscr{L}(\theta)
W = np.random.randn(500,500)/ sqrt(fan_in)However this does not work for ReLU neurons
Why ?
Intuition: He et.al. argue that a factor of 2 is needed when dealing with ReLU Neurons
Intuitively this happens because the range of ReLU neurons is restricted only to the positive half of the space
380 iterations
h_1 \in \mathbb{R}^{100}
h_2,..,h_7 \in \mathbb{R}^{50}
h_1
h_4
h_7
30 iterations/s
Indeed when we account for this factor of 2 we see better performance
W = np.random.randn(fan_in,fan_out)/np.sqrt(fan_in/2)
Module 7.5 : Batch Normalization
Mitesh M. Khapra
AI4Bharat, Department of Computer
Science and Engineering, IIT Madras
We will now see a method called batch normalization which allows us to be less careful about initialization
To understand the intuition behind Batch Normalization let us consider a deep network
Let us focus on the learning process for the weights between these two layers
Typically we use mini-batch algorithms
What would happen if there is a constant change in the distribution of \(h_3\)
In other words what would happen if across minibatches the distribution of \(h_3\) keeps changing
Would the learning process be easy or hard?
x_1
x_2
x_3
h_4
h_3
h_2
h_1
h_0
It would help if the pre-activations at each layer were unit gaussians
h_4
h_3
h_2
h_1
Why not explicitly ensure this by standardizing the pre-activation ?
\hat s_{ i k} = \cfrac {s_{ik}-E[s_{ik}]}{\sqrt{var(s_{ik})}}
But how do we compute \(E[s_{ik}])\) and \(Var[s_{ik}]\)?
We compute it from a mini-batch
Thus we are explicitly ensuring that the distribution of the inputs at different layers does not change across batches
This is what the deep network will look like with Batch Normalization
Is this legal ?
tanh
tanh
tanh
BN
BN
Yes, it is because just as the tanh layer is differentiable, the Batch Normalization layer is also differentiable
Hence we can backpropagate through this layer
Catch: Do we necessarily want to force a unit gaussian input to the tanh layer?
tanh
tanh
tanh
BN
BN
Why not let the network learn what is best for it?
After the Batch Normalization step add the following step:
y^{(k)}=\gamma^k \hat s_{ i k}+\beta^{(k)}
What happens if the network learns:
\gamma^k=\sqrt{var(x^k)}
\beta^k=E[(x^k)]
\( \gamma^k \)and \(\beta^k\) are additional parameters of the network.
We will recover \(s_{ik} \)
In other words by adjusting these additional parameters the network can learn to recover \(s_{ik}\) if that is more favourable
\begin{matrix}
x_1^2\\
x_2^2\\
x_3^2\\
x_4^2\\
x_5^2\\
\end{matrix}
\begin{matrix}
x_1^1\\
x_2^1\\
x_3^1\\
x_4^1\\
x_5^1\\
\end{matrix}
\begin{matrix}
x_1^m\\
x_2^m\\
x_3^m\\
x_4^m\\
x_5^m\\
\end{matrix}
\cdots
Accumulator
Accumulated activations for \(m\) training samples
Let \(x_i^j\) denotes the activation of \(i^{th}\) neuron for \(j^{th}\) training sample
\mu_{i=2} =\frac{1}{m} \sum \limits_{j=1}^{m}x_{2}^j \quad
\sigma_i^2 =\frac{1}{m} \sum \limits_{j=1}^{m}(x_{2}^j-\mu_i)^2 \quad
\hat{x_i} = \frac{x_i-\mu_i}{\sqrt{\sigma_i^2+\epsilon}}
(x_i)
j
l
We have three variables \(l,i, j\) involved in the statistics computation. Let's visualize these as three axes that form a cube.
Let us associate an accumulator with \(l^{th}\) layer that stores the activations of batch inputs.
\hat{y_i} = \gamma \hat{x_i}+\beta
\begin{matrix}
x_1^2\\
x_2^2\\
x_3^2\\
x_4^2\\
x_5^2\\
\end{matrix}
\begin{matrix}
x_1^1\\
x_2^1\\
x_3^1\\
x_4^1\\
x_5^1\\
\end{matrix}
\begin{matrix}
x_1^m\\
x_2^m\\
x_3^m\\
x_4^m\\
x_5^m\\
\end{matrix}
\cdots
Accumulator
Accumulated activations for \(m\) training samples
Let \(x_i^j\) denotes the activation of \(i^{th}\) neuron for \(j^{th}\) training sample
\mu_{i=3} =\frac{1}{m} \sum \limits_{j=1}^{m}x_{3}^j \quad
\sigma_i^2 =\frac{1}{m} \sum \limits_{j=1}^{m}(x_{3}^j-\mu_i)^2 \quad
\hat{x_i} = \frac{x_i-\mu_i}{\sqrt{\sigma_i^2+\epsilon}}
l
We have three variables \(l,i, j\) involved in the statistics computation. Let's visualize these as three axes that form a cube.
Let us associate an accumulator with \(l^{th}\) layer that stores the activations of batch inputs.
(x_i)
j
\hat{y_i} = \gamma \hat{x_i}+\beta
We will now compare the performance with and without batch normalization on MNIST data using 7 layers....
Without Batch Normalization
With Batch Normalization
h_1
h_4
h_7
Blue: Without BN
Red: With BN
Sigmoid Activation, w=0.1*np.random.randn()
Limitations of Batch Normalization
The accuracy of estimation of mean and variance depends on the size of \(m\). So using , due to memory constraint for data like image and videos, a smaller size of \(m\) results in high error.
Because of this, we can't use a batch size of 1 at all (i.e, it won't make any difference, \(\mu_i=x_i,\sigma_i=0\))
Many alternatives were proposed like Group normalization, Instance Normalization which are domain specific. However, the simplest one among them is called Layer Normalization.
Other than this limitation, it was also empirically found that the naive use of BN leads to performance degradation in NLP tasks (source).
There was also a systematic study that validated the statement and proposed a new normalization technique (by modifying BN) called powerNorm.
\mu_{i} =\frac{1}{m} \sum \limits_{j=1}^{m}x_{i}^j \quad
\sigma_i^2 =\frac{1}{m} \sum \limits_{j=1}^{m}(x_{i}^j-\mu_i)^2 \quad
Layer Normalization
The computation is simple. Take the average across outputs of hidden units in the layer. Therefore, the normalization is independent of samples in a batch.
This allows us to work with a batch size of 1 (if needed as in the case of RNN)
\begin{matrix}
x_1\\
x_2\\
x_3\\
x_4\\
x_5\\
\end{matrix}
l^{th} \text{layer}
\sum
\frac{1}{H}
\mu_l=\frac{1}{H}\sum \limits_{i=1}^H x_i
\sigma_l=\sqrt{\frac{1}{H}\sum \limits_{i=1}^H (x_i-\mu_l)^2}
(x_i)
j
l
\hat{x_i} = \frac{x_i-\mu_l}{\sqrt{\sigma_l^2+\epsilon}},\quad \forall i
\hat{y_i} = \gamma \hat{x_i}+\beta