Remove PROCESS GROUP rpc backend (#62411)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/62411
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D29990408
Pulled By: H-Huang
fbshipit-source-id: 183d3b316767b12993cebbe32b73c2850fd1cc42
