[Reland #3] Include AT_PARALLEL_OPENMP/AT_PARALLEL_NATIVE/AT_PARALLEL_NATIVE_TBB to ATen/Config.h (#40122)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/40122
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D22076711
Pulled By: pbelevich
fbshipit-source-id: fa7b6335ebb5ef2ccf51dc60d9f4079e70f612ba
