Document hip-clang and its __HIP__ macro (#16771)
Summary:
In #16085 , we introduced initial hip-clang bring-up code. Document the use of the __HIP__ macro now.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16771
Differential Revision: D13961538
Pulled By: ezyang
fbshipit-source-id: 67f6226abcbe62e2f4efc291c84652199c464ca6
