minimize header file includes from _avx2.cc (#14950)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14950
Minimize the number of headers included from _avx2.cc files to avoid accidental compilation of functions defined the header files reused by other translation units that can lead to illegal instruction errors.
Reviewed By: dskhudia
Differential Revision: D13394483
fbshipit-source-id: 67149a6fb51f7f047e745bfe395cb6dd4ae7c1ae
