Let custom ops registration code only include ATen headers

Summary:
X-link: https://github.com/pytorch/pytorch/pull/107064

Basically we generate `CustomOpsNativeFunctions.h` for registering custom ops into PyTorch JIT runtime. This header needs to hookup with the C++ kernel implementation of all the custom ops. For this reason it should include ATen headers instead of Executorch headers. This PR changes it.

Reviewed By: kirklandsign

Differential Revision: D48282828

fbshipit-source-id: cf4ed96a7fb899b85a71d7efbc3fcd6c5a38632d