Use c10::str in py_rref.cpp (#34681)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/34681
Test Plan: Imported from OSS
Differential Revision: D20428827
Pulled By: mrshenli
fbshipit-source-id: 847486b3114f0e9a2ad5f80c5e44db82d977c6a2
