Enable test_nested_remote in rpc_test.py (#30099)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30099
As after #29827 we only test RPC using spawn, the multi-thread/fork
error should disappear.
Test Plan: Imported from OSS
Differential Revision: D18597003
Pulled By: mrshenli
fbshipit-source-id: ebfb1f6f3f961d98351e06ce4b951793a9b95398
