[Autograd] Cond Higher-Order Operation (pytorch#126911)

This is an updated PR to equip cond with the autograd feature and replaces the old [PR](pytorch#126007)

@ydwu4 I tried to incorporate your requests already.

Currently there are two problems that I struggle with solving:

1. There seems to be an import issue when trying to import cond in `torch/__init__.py`, see [here](https://github.com/bohnstingl/pytorch/blob/8a704035c964acfff35e5b0a2f0acc050895e3eb/torch/__init__.py#L1914-L1916). Therefore, I had to comment those lines, which resolved the import issues, but I believe cond is not proberly exposed as torch.cond.
2. I am not entirely sure how to deal with the opinfo test in `hop_db.py`

Co-authored-by: Yidi Wu <[email protected]>
Pull Request resolved: pytorch#126911
Approved by: https://github.com/ydwu4

Author: 2 people