[numpy] Add `torch.exp2` (#44184)
Summary:
Reference https://github.com/pytorch/pytorch/issues/42515
TODO
* [x] Add tests
* [x] Add docs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44184
Reviewed By: ngimel
Differential Revision: D23674237
Pulled By: mruberry
fbshipit-source-id: 7f4fb1900fad3051cd7fc9d3d7f6d985c5fb093c
