Set default ATen threading backend to native if USE_OPENMP is false (#43067)
Summary:
Since OpenMP is not available on some platforms, or might be disabled by user, set default `ATEN_THREADING` based on USE_OPENMP and USE_TBB options
Fixes https://github.com/pytorch/pytorch/issues/43036
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43067
Reviewed By: houseroad
Differential Revision: D23138856
Pulled By: malfet
fbshipit-source-id: cc8f9ee59a5559baeb3f19bf461abbc08043b71c
