## Hot questions for Using Neural networks in deep residual networks

Question:

Following is the Pytorch implement of RestNet-18: https://github.com/pytorch/vision/blob/master/torchvision/models/resnet.py And following is the structure of ResNet-18, anyone knows why this net has 18 layers?

ResNet ( (conv1): Conv2d(3, 64, kernel_size=(7, 7), stride=(2, 2), padding=(3, 3), bias=False) (bn1): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True) (relu): ReLU (inplace) (maxpool): MaxPool2d (size=(3, 3), stride=(2, 2), padding=(1, 1), dilation=(1, 1)) (layer1): Sequential ( (0): BasicBlock ( (conv1): Conv2d(64, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn1): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True) (relu): ReLU (inplace) (conv2): Conv2d(64, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn2): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True) ) (1): BasicBlock ( (conv1): Conv2d(64, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn1): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True) (relu): ReLU (inplace) (conv2): Conv2d(64, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn2): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True) ) ) (layer2): Sequential ( (0): BasicBlock ( (conv1): Conv2d(64, 128, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1), bias=False) (bn1): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True) (relu): ReLU (inplace) (conv2): Conv2d(128, 128, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn2): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True) (downsample): Sequential ( (0): Conv2d(64, 128, kernel_size=(1, 1), stride=(2, 2), bias=False) (1): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True) ) ) (1): BasicBlock ( (conv1): Conv2d(128, 128, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn1): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True) (relu): ReLU (inplace) (conv2): Conv2d(128, 128, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn2): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True) ) ) (layer3): Sequential ( (0): BasicBlock ( (conv1): Conv2d(128, 256, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1), bias=False) (bn1): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True) (relu): ReLU (inplace) (conv2): Conv2d(256, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn2): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True) (downsample): Sequential ( (0): Conv2d(128, 256, kernel_size=(1, 1), stride=(2, 2), bias=False) (1): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True) ) ) (1): BasicBlock ( (conv1): Conv2d(256, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn1): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True) (relu): ReLU (inplace) (conv2): Conv2d(256, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn2): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True) ) ) (layer4): Sequential ( (0): BasicBlock ( (conv1): Conv2d(256, 512, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1), bias=False) (bn1): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True) (relu): ReLU (inplace) (conv2): Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn2): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True) (downsample): Sequential ( (0): Conv2d(256, 512, kernel_size=(1, 1), stride=(2, 2), bias=False) (1): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True) ) ) (1): BasicBlock ( (conv1): Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn1): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True) (relu): ReLU (inplace) (conv2): Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False) (bn2): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True) ) ) (avgpool): AvgPool2d ( ) (fc): Linear (512 -> 1000) )

Answer:

From your output, we can know that there are 20 convolution layers (one 7x7 conv, 16 3x3 conv, and plus 3 1x1 conv for downsample). Basically, if you ignore the 1x1 conv, and counting the fc(linear) layer, the number of layers are 18.

And I've also made a example https://gist.github.com/wangg12/f11258583ffcc4728eb71adc0f38e832 on how to visualize your architecture in pytorch via graphviz, hope it will help you understand your architecture.

Question:

I am reading through Residual learning, and I have a question. What is "linear projection" mentioned in 3.2? Looks pretty simple once got this but could not get the idea...

I am basically not a computer science person, so I would very appreciate if someone provide me a simple example.

Answer:

First up, it's important to understand what `x`

, `y`

and `F`

are and why they need any projection at all. I'll try explain in simple terms, but basic understanding of ConvNets is required.

`x`

is an input data (called *tensor*) of the layer, in case of ConvNets it's rank is 4. You can think of it as a 4-dimensional array. `F`

is usually a conv layer (`conv+relu+batchnorm`

in this paper), and `y`

combines the two together (forming the output channel). The result of `F`

is also of rank 4, and most of dimensions will be the same as in `x`

, except for one. That's exactly what the transformation should patch.

For example, `x`

shape might be `(64, 32, 32, 3)`

, where 64 is the batch size, 32x32 is image size and 3 stands for (R, G, B) color channels. `F(x)`

might be `(64, 32, 32, 16)`

: batch size never changes, for simplicity, ResNet conv-layer doesn't change the image size too, but will likely use a different number of filters - 16.

So, in order for `y=F(x)+x`

to be a valid operation, `x`

must be "reshaped" from `(64, 32, 32, 3)`

to `(64, 32, 32, 16)`

.

**I'd like to stress here that "reshaping" here is not what numpy.reshape does.**

Instead, `x[3]`

is padded with 13 zeros, like this:

pad(x=[1, 2, 3],padding=[7, 6]) = [0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 0, 0, 0, 0, 0, 0]

If you think about it, this is a projection of a 3-dimensional vector onto 16 dimensions. In other words, we start to think that our vector is the same, but there are 13 more dimensions out there. None of the other `x`

dimensions are changed.

Here's the link to the code in Tensorflow that does this.

Question:

I've looked everywhere and can't find anything that explains the actual derivation of backprop for residual layers. Here's my best attempt and where I'm stuck. It is worth mentioning that the derivation that I'm hoping for is from a generic perspective that need not be limited to convolutional NNs.

If the formula for calculating the output of a normal hidden layer is F(x) then the formula for a hidden layer with a residual connection is F(x) + o, where x is the weight adjusted output of a previous layer, o is the output of a previous layer, and F is the activation function. To get the delta for a normal layer during back-propagation one needs to calculate the gradient of the output ∂F(x)/∂x. For a residual layer this is ∂(F(x) + o)/∂x which is separable into ∂F(x)/∂x + ∂o/∂x (1).

If all of this is correct, how does one deal with ∂o/∂x? It seems to me that it depends on how far back in the network o comes from.

If o is just from the previous layer then o*w=x where w are the weights connecting the previous layer to the layer for F(x). Taking the derivative of each side relative to o gives ∂(o*w)/∂o = ∂x/∂o, and the result is w = ∂x/do which is just the inverse of the term that comes out at (1) above. Does it make sense that in this case the gradient of the residual layer is just ∂F(x)/∂x + 1/w ? Is it accurate to interpret 1/w as a matrix inverse? If so then is that actually getting computed by NN frameworks that use residual connections or is there some shortcut that is for adding in the error from the residual?

If o is from further back in the network then, I think, the derivation becomes slightly more complicated. Here is an example where the residual comes from one layer further back in a network. The network architecture is Input--w1--L1--w2--L2--w3--L3--Out, having a residual connection from the L1 to L3 layers. The symbol o from the first example is replaced by the layer output L1 for unambiguity. We are trying to calculate the gradient at L3 during back-prop which has a forward function of F(x)+L1 where x=F(F(L1*w2)*w3). The derivative of this relationship is ∂x/∂L1=∂F(F(L1*w2)*w3/∂L1, which is more complicated but doesn't seem too difficult to solve numerically.

If the above derivation is reasonable then it's worth noting that there is a case when the derivation fails, and that is when a residual connection originates from the Input layer. This is because the input cannot be broken down into a o*w=x expression (where x would be the input values). I think this must suggest that residual layers cannot originate from from the input layer, but since I've seen network architecture diagrams that have residual connections that originate from the input, this casts my above derivations into doubt. I can't see where I've gone wrong though. If anyone can provide a derivation or code sample for how they calculate the gradient at residual merge points correctly, I would be deeply grateful.

EDIT:

The core of my question is, when using residual layers and doing vanilla back-propagation, is there any special treatment of the error at the layers where residuals are added? Since there is a 'connection' between the layer where the residual comes from and the layer where it is added, does the error need to get distributed backwards over this 'connection'? My thinking is that since residual layers provide raw information from the beginning of the network to deeper layers, the deeper layers should provide raw error to the earlier layers.

Based on what I've seen (reading the first few pages of googleable forums, reading the essential papers, and watching video lectures) and Maxim's post down below, I'm starting to think that the answer is that ∂o/∂x = 0 and that we treat o as a constant.

Does anyone do anything special during back-prop through a NN with residual layers? If not, then does that mean residual layers are an 'active' part of the network on only the forward pass?

Answer:

I think you've over-complicated residual networks a little bit. Here's the link to the original paper by Kaiming He at al.

In section 3.2, they describe the "identity" shortcuts as `y = F(x, W) + x`

, where `W`

are the trainable parameters. You can see why it's called "identity": the value from the previous layer is added *as is*, without any complex transformation. This makes two things:

`F`

now learns the residual`y - x`

(discussed in 3.1), in short: it's easier to learn.- The network gets an extra connection to the previous layer, which improves the gradients flow.

The backward flow through the identity mapping is trivial: the error message is passed unchanged, no inverse matrices are involved (in fact, they are not involved in any linear layer).

Now, paper authors go a bit further and consider a slightly more complicated version of `F`

, which changes the output dimensions (which probably you had in mind). They write it generally as `y = F(x, W) + Ws * x`

, where `Ws`

is the *projection matrix*. Note that, though it's written as matrix multiplication, this operation is in fact very simple: it adds extra zeros to `x`

to make its shape larger. You can read a discussion of this operation in this question. But this does very few changes the backward: the error message is simply clipped to the original shape of `x`

.

Question:

Standard in ResNets is to skip 2 linearities. Would skipping only one work as well?

Answer:

I would refer you to the original paper by Kaiming He at al.

In sections 3.1-3.2, they define "identity" shortcuts as `y = F(x, W) + x`

, where `W`

are the trainable parameters, **for any residual mapping** `F`

to be learned. It is important that the residual mapping contains a *non-linearity*, otherwise the whole construction is one sophisticated linear layer. But the number of linearities is not limited.

For example, ResNeXt network creates identity shortcuts around a stack of only convolutional layers (see the figure below). So there aren't *any* dense layers in the residual block.

The general answer is, thus: yes, it would work. However, in a particular neural network, reducing two dense layers to one may be a bad idea, because anyway the residual block must be flexible enough to learn the residual function. So remember to validate any design you come up with.

Question:

Like the title says, is there any difference between the two? I think the original inception v1 model does not have Res Blocks, but maybe I'm wrong. Are they the same thing?

Answer:

No, they are not the same?

Inception-ResNet is hybrid module inspired both by inception and the performance of resnet. There are further two sub-versions of Inception-ResNet, namely v1 and v2. Inception-ResNet v2 has higher accuracy and computational cost as compared to Inception-ResNet v1 (https://ai.googleblog.com/2016/08/improving-inception-and-image.html).