[JIT] Adjust pybind includes in backend.h (#38562)
Summary:
**Summary**
This commit adjusts the `pybind` includes in `backend.h` so
that we can avoid exporting some unrelated headers during install (which
probably shouldn't be exposed anyway). In addition, the headers that this commit
removes are not used.
**Test Plan**
Continuous integration (includes tests for JIT backends).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38562
Differential Revision: D21601694
Pulled By: SplitInfinity
fbshipit-source-id: c8f8103d24cb4f10d9eb6b3657eed75878078945
