Adding parallel support for the LLVM backend. (#53243)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/53243
Test Plan: Imported from OSS
Reviewed By: bertmaher, Chillee
Differential Revision: D26906509
Pulled By: zheng-xq
fbshipit-source-id: 12c17f2f21af11e73fa4c5b5199043a7a15ecdec
