How is im2col()'s backward being called?

I know that im2col()'s backward is col2im(), and I’m studying how F.unfold() triggers the backward pass of im2col().

I noticed there are functions like im2col_backward_cuda()/im2col_backward_out_cuda()/im2col_backward_out_cuda_template() in Pytorch repository. However, when searching for these names, the only hit is the file itself.

My question is:

  1. Are the above backward functions ever called at all?
  2. How is im2col()'s backward triggered?

Thank you!