[special] Add `i1` and `i1e` (#56352)
Summary:
Reference: https://github.com/pytorch/pytorch/issues/50345
* [x] Check Docs https://12721710-65600975-gh.circle-artifacts.com/0/docs/special.html
* [x] Investigate fp32 failure on CI?! (Fails on clang. Reproduced locally with clang-11)
* [ ] Kernel vs Composite?
* [x] Autograd for `i0e` for zero?
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56352
Reviewed By: anjali411
Differential Revision: D28700888
Pulled By: mruberry
fbshipit-source-id: 91a3cbb94f5b8a3b063589ec38179848c11def83
