Allow a non-OpenMP based build (#19749)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19749
ghimport-source-id: a6636c0acddbdc5fd5b0dcb20b9f80cbdb9159b9
Differential Revision: D15141993
Pulled By: ilia-cher
fbshipit-source-id: 96085608398b2a4c97c68b2948f5184d07f9ad3d
