How PyTorch implements Convolution Backward?
I read about the Pytorch's source code, and I find it's weird that it doesn't implement the convolution_backward function, The only convolution_backward_overrideable function is directly raises an error and supposed not to fall here.
So I referred to CuDNN / MKLDNN implementation, they both implements functions like cudnn_convolution_backward.
I got the following question:

What are the native implementation of CUDA/ CPU? I can find something like thnn_conv2d_backward_out, but I could not find where are this is called.

Why PyTorch didn't put the convolution_backward function in Convolution.cpp? It offers an _convolution_double_backward() function. But this is the double backward, it's the gradient of gradient. Why don't they offer a single backward function?

If I want to call the native convolution/ convolution_backward function for my pure cpu/cuda tensor, how should I write code? Or where could I refer to? I couldn't find example for this.


Thanks !

1- Implementation may differ depending on which backend you use, it may use CUDA convolution implementation from some library, CPU convolution implementation from some other library, or custom implementation, see here: pytorch - Where is “conv1d” implemented?.
2- I am not sure about the current version, but single backward was calculated via autograd, that is why there was not an explicit different function for it. I don't know the underlying details of autograd but you can check https://github.com/pytorch/pytorch/blob/master/torch/csrc/autograd/autograd.cpp. That double_backward function is only there if you need higher order derivatives.
3- If you want to do this in C, the file you linked (convolution.cpp) shows you how to do this (function at::Tensor _convolution...). If you inspect the function you see it just checks which implementation to use (params.use_something...) and use it. If you want to do this in python you should start tracing from conv until where this file convolution.cpp is called.
