Add return_counts to torch.unique (#18391)

Add return_counts to torch.unique (#18391)

Summary:
Fixes: https://github.com/pytorch/pytorch/issues/12598

This PR was originally authorized by ptrblck at https://github.com/pytorch/pytorch/pull/15495, but since there was no update for months after the request change, I clone that branch and resolve the code reviews here. Hope everything is good now. Especially, the implementation of count is changed from ptrblck's original algorithm to the one ngimel suggest, i.e. using `unique_by_key` and `adjacent_difference`.

The currently implementation of `_unique_dim` is VERY slow for computing inverse index and counts, see https://github.com/pytorch/pytorch/issues/18405. I will refactor `_unique_dim` in a later PR. For this PR, please allow me to keep the implementation as is.

cc: ptrblck ezyang ngimel colesbury
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18391

Reviewed By: soumith

Differential Revision: D14605905

Pulled By: VitalyFedyunin

fbshipit-source-id: 555f5a12a8e28c38b10dfccf1b6bb16c030bfdce