Remove process_group_agent and faulty_process_group_agent files (#62985)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62985
Remove the process_group_agent and faulty_process_group_agent code now that PROCESS_GROUP backend has been deprecated for RPC (https://github.com/pytorch/pytorch/issues/55615). Discussed with xush6528 that it was okay to remove ProcessGroupAgentTest and ProcessGroupAgentBench which depended on process_group_agent.
Test Plan: CI tests
Reviewed By: pritamdamania87
Differential Revision: D30195576
fbshipit-source-id: 8b4381cffadb868b19d481198015d0a67b205811
