This note is divided into 4 section.
 Section1: What is the meaning of ‘deeplearning from foundations?’
 Section2: What’s inside Pytorch Operator?
 Section3: Implement forward&backward pass from scratch
 Section4: Gradient backward, Chain Rule, Refactoring
” Lecture 08  Deep Learning From Foundationspart2 “
Homework
CONTENTS
 Forward process
Foundation version
Gradients backward pass
 Gradients is output with respect to parameter
 we’ve done this work in this path(below)
 to simplify this calculus, we can just change it into
,
 So, you should know of the derivative of each bit on its own, and then you multiply them all together. As a result, it would be over cross over the data.
 So you can get gradient, output with respect to parameter
 What order should we calculate?
BTW, why Jeremy wrote , not Loss function?^{1}
decompose function
 We want to get derivative of which forms
 But, we have a estimation of answer (we call it y hat) now
 So, I will decompose funciton to trace target variable.
 Using the above forward pass, we can suppose some function from the end.
 start from , We know MSE funciton got two parameters, output, and target .
 from MSE’s input we know function’s output and supposing v is input of that function,
 similarly, v became output of
chain rule with code

examplify backward process by random sampling

To get a variable, I modified forward model a little
def model_ping(out = 'x_train'):
l1 = lin(x_train, w1, b1) # one linear layer
l2 = relu(l1) # one relu layer
l3 = lin(l2, w2, b2) # one more linear layer
return eval(out)
 Be careful we don’t use mse_loss in backward process
1) start with the very last function, which is loss funciton. MSE
 If we codify this formula,
def mse_grad(inp, targ): #mse_input(1000,1), mse_targ (1000,1)
# grad of loss with respect to output of previous layer
inp.g = 2. * (inp.squeeze()  targ).unsqueeze(1) / inp.shape[0]
 And, this can be examplified like below.
 Notice that input of gradient function is same with forward function
y_hat = model_ping('l3') #get value from forward model
y_hat.g = ((y_hat.squeeze(1)y_train).unsqueeze(1))/y_hat.shape[0]
y_hat.g.shape
>>> torch.Size([50000, 1])
 We can just calculate using broadcasting, not using squeeze. then why should do and unsqueeze again?
🎯 It’s related with random access memory(RAM).. If I don’t squeeze, (I’m using colab) it out of RAM.
2) Derivative of linear2 function
 This process’s weight dimensions defined by axis=1, axis=2.
 axis=0 dimension means size of data. This will be summazed by .sum(0) method.
 unsqeeze(1)&unsqeeze(1) seperates the dimension, and make a dot product, and vanish axis=0 dimension.
def lin_grad(inp, out, w, b):
# grad of matmul with respect to input
inp.g = out.g @ w.t()
w.g = (inp.unsqueeze(1) * out.g.unsqueeze(1)).sum(0)
b.g = out.g.sum(0)
 Examplified below
lin2 = model_ping('l2'); #get value from forward model
lin2.g = y_hat.g@w2.t();
w2.g = (lin2.unsqueeze(1) * y_hat.g.unsqueeze(1)).sum(0);
b2.g = y_hat.g.sum(0);
lin2.g.shape, w2.g.shape, b2.g.shape
>>> torch.Size([50000, 50])torch.Size([50, 1])torch.Size([1])
 Notice going reverse order, we’re passing in gradient backward
3) derivative of ReLU
def relu_grad(inp, out):
# grad of relu with respect to input activations
inp.g = (inp>0).float() * out.g
 Examplified below
lin1=model_ping('l1') #get value from forward model
lin1.g = (lin1>0).float() * lin2.g;
lin1.g.shape
>>> torch.Size([50000, 50])
4) Derivative of linear1
 Same process with 2) but, this process’s weight has
def lin_grad(inp, out, w, b):
# grad of matmul with respect to input
inp.g = out.g @ w.t()
w.g = (inp.unsqueeze(1) * out.g.unsqueeze(1)).sum(0)
b.g = out.g.sum(0)
 Examplified below
x_train.g = lin1.g @ w1.t();
w1.g = (x_train.unsqueeze(1) * lin1.g.unsqueeze(1)).sum(0);
b1.g = lin1.g.sum(0);
x_train.g.shape, w1.g.shape, b1.g.shape
>>> torch.Size([50000, 784])torch.Size([784, 50])torch.Size([50])
5) Then it goes backward pass
def forward_and_backward(inp, targ):
# forward pass:
l1 = inp @ w1 + b1
l2 = relu(l1)
out = l2 @ w2 + b2
# we don't actually need the loss in backward!
loss = mse(out, targ)
# backward pass:
mse_grad(out, targ)
lin_grad(l2, out, w2, b2)
relu_grad(l1, l2)
lin_grad(inp, l1, w1, b1)
Version 1 (Basic) Wall time: 1.95 s
Summary
 Notice that output of function at forward pass became input of backward pass
 backpropagation is just the chain rule
 value loss (loss=mse(out,targ)) is not used in gradient calcuation.
 Because, it doesn’t appear with the weight.
 w1g, w2g, b1g, b2g, ig will be used for optimizer
check the result using Pytorch autograd
 require_grad_ is the magical function, which can automatic differentiation.^{2}
 This magical auto gradified tensor keep track what happend in forward (taking loss function),
 and do the backward^{3}
 So it saves our time to differentiate ourselves
 Postfix underscore means in pytorch,
inplace
function, What is inplace function?
⤵️ THis is benchmark…..
Version 2 (torch autograd) Wall time: 3.81 µs
Refactor model
 Amazingly, just refactoring our main pieces, it comes down up to Pytorch package.
🌟 Implement yourself, Practice, practice, practice! 🌟
Layers as classes

Relu and Linear are layers in oue neural net. > make it as classes

For the forward, using __call__ for the both of forward & backward. Because ‘call’ means we treat this as a function.
class Lin():
def __init__(self, w, b): self.w,self.b = w,b
def __call__(self, inp):
self.inp = inp
self.out = inp@self.w + self.b
return self.out
def backward(self):
self.inp.g = self.out.g @ self.w.t()
# Creating a giant outer product, just to sum it, is inefficient!
self.w.g = (self.inp.unsqueeze(1) * self.out.g.unsqueeze(1)).sum(0)
self.b.g = self.out.g.sum(0)
 Remember that in lin_grad function, we save bias&weight!!!!!
💬 inp.g : gradient of the output with respect to the input.
{: style=”color:grey; fontsize: 90%; textalign: center;”}
💬 w.g : gradient of the output with respect to the weight.
{: style=”color:grey; fontsize: 90%; textalign: center;”}
💬 b.g : gradient of the output with respect to the bias.
{: style=”color:grey; fontsize: 90%; textalign: center;”}
class Model():
def __init__(self, w1, b1, w2, b2):
self.layers = [Lin(w1,b1), Relu(), Lin(w2,b2)]
self.loss = Mse()
def __call__(self, x, targ):
for l in self.layers: x = l(x)
return self.loss(x, targ)
def backward(self):
self.loss.backward()
for l in reversed(self.layers): l.backward()

refer to Jeremy’s Model class, he put layers in list
 Dionne’s selfstudy note: Decomposing Jeremy’s Model class
 init needs weight, bias but not x data
 when call that class(a.k.a function) it gave x data and y label!
 jeremy composited function in layers. x = l(x) so concise…..
 also utilized that layer list when backward ust reversing it (using python list’s method)
 And he is recursively calling the function on the result of the previous thing. ⬇️
for l in self.layers:
x = l(x)
Q2: Don’t I need to declare magical autograd function, requires_grad_?{: style=”color:red; fontsize: 130%; textalign: center;”}
[The questions migrated to this article]
Version 3 (refactoring  layer to class) Wall time: 5.25 µs
Modue.forward()
 Duplicate code makes execution time slow.
 Role of
__call__
changed. No more__call__
for implementing forward pass.  By initializing the forward with
__call__
, Module.forward() use overriding to maximize reusability. So any layer inherit Module, can use parent’s function.
 Role of
 gradient of the output with respect to the weight
(self.inp.unsqueeze(1) * self.out.g.unsqueeze(1)).sum(0)
can be reexpressed using einsum,
torch.einsum("bi,bj>ij", inp, out.g)
 Defining forward and Module enables Pytorch to out almost duplicates
Version 4 (Module & einsum) Wall time: 4.29 µs
Q2: Isn’t there any way to use broadcasting? Why we should use outer product?{: style=”color:red; fontsize: 130%; textalign: center;”}
Without einsum
Replacing einsum to matrix product is even more faster.
torch.einsum("bi,bj>ij", inp, out.g)
can be reexpressed using matrix product,
inp.t() @ out.g
Version 5 (without einsum) Wall time: 3.81 µs
nn.Linear and nn.Module
Torch’s package nn.Linear and nn.Module
Version 6 (torch package) Wall time: 5.01 µs
 Final, Using torch.nn.Linear & torch.nn.Module ~~~python
class Model(nn.Module): def init(self, n_in, nh, n_out): super().init() self.layers = [nn.Linear(n_in,nh), nn.ReLU(), nn.Linear(nh,n_out)] self.loss = mse
def __call__(self, x, targ):
for l in self.layers: x = l(x)
return self.loss(x.squeeze(), targ)
class Model(): def init(self): self.layers = [Lin(w1,b1), Relu(), Lin(w2,b2)] self.loss = Mse()
def __call__(self, x, targ):
for l in self.layers: x = l(x)
return self.loss(x, targ)
def backward(self):
self.loss.backward()
for l in reversed(self.layers): l.backward()
~~~