Change hip filename extension to .hip (#14036)

Change hip filename extension to .hip (#14036)

Summary:
xw285cornell

- To make hip files to have unique filename extension we change hip files from _hip.cc to .hip (it's the only blessing option other than .cu in hipcc https://github.com/ROCm-Developer-Tools/HIP/blob/3d51a1fb0105e2f2312d2523c20e0034339f6ada/bin/hipcc#L552).
- Change to use host compiler to compile .cc|.cpp files. Previously we use hcc to compile them which is unnecessary
- Change the hipify script to not replace "gpu" with "hip" in the filename of the generated hipified files. Previously we do this because hcc has a bug when linking files that have same filename. We have now changed to use host linker to do linking so this is unnecessary anymore.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14036

Reviewed By: xw285cornell

Differential Revision: D13091813

Pulled By: bddppq

fbshipit-source-id: ea3d887751d8abb39d75f5d5104aa66ce66b9ee0