Consolidate the places that find pybind11 include dirs (#29659)
Summary:
Also move the logic that installs the pybind11 headers from setup.py to cmake (to align with other headers).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29659
Differential Revision: D18458208
Pulled By: bddppq
fbshipit-source-id: cfd1e74b892d4a65591626ab321780c8c87b810d
