Enable test_nested_rpc in rpc_test.py (#30098)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30098
As after #29827 we only test RPC using spawn, the multi-thread/fork
error should disappear.
Test Plan: Imported from OSS
Differential Revision: D18597001
Pulled By: mrshenli
fbshipit-source-id: 68256289085fac1a9ca76d5b4882e97e2f81d1f4
