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
1. The forward and backward passes
1.1 Normalization
train_mean,train_std = x_train.mean(),x_train.std()
>>> train_mean,train_std
(tensor(0.1304), tensor(0.3073))
Remember!
 Dataset, which is x_train, mean and standard deviation is not 0&1. But we need them to be which means we should substract means and divide data by std.
 You should not standarlize validation set because training set and validation set should be aparted.
 after normalize, mean is close to zero, and standard deviation is close to 1.
1.2 Variable definition
 n,m: size of the training set
 c: the number of activations we need in our model
2. Foundation Version
2.1 Basic architecture
 Our model has one hidden layer, output to have 10 activations, used in cross entropy.

But in process of building architecture, we will use mean square error, output to have 1 activations and lator change it to cross entropy
 number of hidden unit; 50
see below pic
 We want to make w1&w2 mean and std be 0&1.
 why initializating and make mean zero and std one is important?
 paper highlighting importance of normalisation  training 10,000 layer network without regularisation^{1}
2.1.1 simplified kaiming init
Q: Why we did init, normalize with only validation data? Because we can not handle and get statistics from each value of x_valid?{: style=”color:red; fontsize: 130%; textalign: center;”}
 what about hidden(first) layer?
w1 = torch.randn(m,nh)
b1 = torch.zeros(nh)
t = lin(x_valid, w1, b1) # hidden
>>> t.mean(), t.std()
((tensor(2.3191), tensor(27.0303))
In output(second) layer,
w2 = torch.randn(nh,1)
b2 = torch.zeros(1)
t2 = lin(t, w2, b2) # output
>>> t2.mean(), t2.std()
(tensor(58.2665), tensor(170.9717))

which is terribly far from normalzed value.

But if we apply simplified kaiming init
w1 = torch.randn(m,nh)/math.sqrt(m); b1 = torch.zeros(nh)
w2 = torch.randn(nh,1)/math.sqrt(nh); b2 = torch.zeros(1)
t = lin(x_valid, w1, b1)
t.mean(),t.std()
>>> (tensor(0.0516), tensor(0.9354))
 But, actually, we use activations not only linear function
 After applying activations relu at linear layer, mean and deviation became 0.5.
2.1.2 Glorrot initialization
Paper2: Understanding the difficulty of training deep feedforward neural networks
 Gaussian(, bell shaped, normal distributions) is not trained very well.
 How to initialize neural nets?
with the size of layer , the number of filters .
 But there is No acount for import of ReLU
 If we got 1000 layers, vanishing gradients problem emerges
2.1.3 Kaiming initializating
Paper3: Delving Deep into Rectifiers: Surpassing HumanLevel Performance on ImageNet Classification
 Kaiming He, explained here
 rectifier: rectified linear unit
 rectifier network: neural network with rectifier linear units
 This is kaiming init, and why suddenly replace one to two on a top?
 to avoid vanishing gradient(weights)
 But it doesn’t give very nice mean tough.
2.1.4 Pytorch package
 Why fan_out?
 according to pytorch documentation,
choosing 'fan_in' preserves the magnitude of the variance of the wights in the forward pass.
choosing 'fan_out' preserves the magnitues in the backward pass(, which means matmul; with transposed matrix)
➡️ in the other words, torch use fan_out cz pytorch transpose in linear transformaton.
 What about CNN in Pytorch?
I tried
torch.nn.Conv2d.conv2d_forward??
Jeremy digged into using
torch.nn.modules.conv._ConvNd.reset_parameters??
^{2}
 in Pytorch, it doesn’t seem to be implemented kaiming init in right formula. so we should use our own operation.
 But actually, this has been discussed in Pytorch community before.^{3} ^{4}
 Jeremy said it enhanced variance also, so I sampled 100 times and counted better results.
 To make sure the shape seems sensible. check with assert. (remember we will replace 1 to 10 in cross entropy)
assert model(x_valid).shape==torch.Size([x_valid.shape[0],1])
>>> model(x_valid).shape
(10000, 1)
 We have made Relu, init, linear, it seems we can forward pass
 code we need for basic architecture
nh = 50
def lin(x, w, b): return x@w + b;
w1 = torch.randn(m,nh)*math.sqrt(2./m ); b1 = torch.zeros(nh)
w2 = torch.randn(nh,1); b2 = torch.zeros(1)
def relu(x): return x.clamp_min(0.)  0.5
t1 = relu(lin(x_valid, w1, b1))
def model(xb):
l1 = lin(xb, w1, b1)
l2 = relu(l1)
l3 = lin(l2, w2, b2)
return l3
2.2 Loss function: MSE
 Mean squared error need unit vector, so we remove unit axis.
def mse(output, targ): return (output.squeeze(1)  targ).pow(2).mean()
 In python, in case you remove axis, you use ‘squeeze’, or add axis use ‘unsqueeze’
 torch.squeeze where code commonly broken. so, when you use squeeze, clarify dimension axis you want to remove
tmp = torch.tensor([1,1])
tmp.squeeze()
>>> tensor([1, 1])
 make sure to make as float when you calculate
But why??? because it is tensor?{: style=”color:red; fontsize: 130%;”}
Here’s the error when I don’t transform the data type

TypeError Traceback (most recent call last)
<ipythoninput22ae6009bef8b4> in <module>()
> 1 y_train = get_data()[1] # call data again
2 mse(preds, y_train)
TypeError: 'map' object is not subscriptable
 This is forward pass
Footnote
Other materials
 Understanding the difficulty of training deep feedforward neural networks, paper that introduced Xavier initialization