[executorch] Let custom ops registration code only import ATen headers (#107064)
Summary: 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.
Test Plan: Rely on existing CI jobs
Differential Revision: D48282828
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107064
Approved by: https://github.com/kirklandsign
