Remove backward for im2col and col2im (#85542)
`im2col` is a linear map, and `col2im` is its adjoint. As such, the
adjoint to `col2im` is `im2col` (the adjoint of the adjoint is the
original function.
There's no point having explicit derivatives in ATen for these
functions, so this PR deletes all these.
Furthermore, along the way, we fix an error for the derivative of im2col
for non-batched inputs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85542
Approved by: https://github.com/soulitzer, https://github.com/ngimel
