Add refs for torch.unfold and a decomposition for its backward. (#85629)
It's not clear to me what's the difference between `unfold` and `unfold_copy`, as this latter one is codegen'd
I also took this chance to clean the implementation of unfold and its reference
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85629
Approved by: https://github.com/mruberry
