Split ATen/Parallel into interface and backend (#20057)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20057
ghimport-source-id: c583f61bf661c994eb4d0625748a299e892a7246
Differential Revision: D15248618
Pulled By: ilia-cher
fbshipit-source-id: 060879266bc8616916fe220adef6ae6c0b076fbd
