Use c10::str in process_group_agent.cpp (#34679)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/34679
Test Plan: Imported from OSS
Differential Revision: D20428467
Pulled By: mrshenli
fbshipit-source-id: 2bfde4e383347c6e709109f074f55b9bc8068a49
