Refactor NnapiCompilation registration into it's own file (#63183)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63183
Move registration of NnapiCompilation into it's own file, so that `nnapi_bind.cpp` (which contains the implementation of NnapiCompilation) can be moved to `aten_cpu`, while maintaining the selectiveness for registration.
`nnapi_bind.cpp` is moved to `aten_cpu` in https://github.com/pytorch/pytorch/pull/62919. See the PR for more details on why it's needed.
ghstack-source-id: 135900318
Test Plan: Nnapi unit tests: `python test/test_nnapi.py`
Reviewed By: iseeyuan
Differential Revision: D30288708
fbshipit-source-id: 6ed5967fa6bd018075469d18e68f844d413cf265
