Enable test_nested_rref in rpc_test.py (#30100)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30100
As after #29827 we only test RPC using spawn, the multi-thread/fork
error should disappear.
Test Plan: Imported from OSS
Differential Revision: D18597002
Pulled By: mrshenli
fbshipit-source-id: 64aa6a59248e5d1b7e1ad1aebffb6a25248388d2
