add bfloat16 impl for nextafter (#61829)
Summary:
Add `BFloat16` support for `nextafter`.
* [x] Add OpInfo
* [x] Add Implementation Test (C++ tests)
* [x] Add credit
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61829
Reviewed By: ejguan
Differential Revision: D29932498
Pulled By: mruberry
fbshipit-source-id: 89524531a4800569ba1addd08a4ace330a6f72a4
