Refactor NnapiCompilation registration into it's own file (#63183)

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