This might sound a little basic but while running the code below, I wanted to see the source code of the backward function:

import torch.nn as nn

criterion = nn.CrossEntropyLoss()
loss = criterion(output, target)

So I went to the PyTorch GitHub and found the CrossEntropyLoss class, but without any backward function defined. Moving up, CrossEntropyLoss extends _WeightedLoss >> _Loss >> Module then still nothing.

So, where is the backward function defined?


Backward function is same for all type of layers.

Look at 155 number line here.


and it will forward you to here


  • 1
    $\begingroup$ criterion return a Tensor... oh... i missed that part. Thanks a lot! $\endgroup$
    – Begoodpy
    Jul 8 '20 at 17:43

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