Fix docstring to clarify logits usage for multiclass case (#51053)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/50378.
Additionally, this has some minor fixes:
- [x] Fix mean for half-cauchy to return `inf` instead of `nan`.
- [x] Fix constraints/support for the relaxed categorical distribution.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51053
Reviewed By: heitorschueroff
Differential Revision: D26077966
Pulled By: neerajprad
fbshipit-source-id: ca0213baa9bbdbc661aebbb901ab5e7fded38a5f
