[numpy] Add `torch.xlogy` (#48777)
Summary:
Reference https://github.com/pytorch/pytorch/issues/38349
Fixes https://github.com/pytorch/pytorch/issues/22656
TODO:
* [x] Add docs
* [x] Add tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48777
Reviewed By: ngimel
Differential Revision: D25681346
Pulled By: mruberry
fbshipit-source-id: 369e0a29ac8a2c44de95eec115bf75943fe1aa45
