[special] add `torch.special` namespace (#52296)
Summary:
Reference: https://github.com/pytorch/pytorch/issues/50345
* Add `torch.special` namespace
* Add `torch.special.gammaln` (alias to `torch.lgamma`)
TODO:
* Add proper entries for docs.
* [x] Add .rst file entry
* [x] Add documentation
* [x] Update `lgamma` OpInfo entry for alias to `special.gammaln`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52296
Reviewed By: ngimel
Differential Revision: D26754890
Pulled By: mruberry
fbshipit-source-id: 73479f68989d6443ad07b7b02763fa98973c15f6
