Resend "Split ATen/Parallel into interface and backend" (#20825)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20825
ghimport-source-id: 0371fbd37cb37635647d473d5ac9f2859e787061
Differential Revision: D15458073
Pulled By: ilia-cher
fbshipit-source-id: cd27d0da1691f6be1183cd152348ac0d93a53996
