Defer pg agent listener thread until contexts are initialized (#28013)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28013
ProcessGroupAgent currently kicks off the listener thread in its
constructor. However, serving requests requires contexts to be
initialized, e.g., RRefContext and agent_ global var in api.py,
which might not be done yet when the first request arrives.
ProcessGroupAgent does not know what would be the appropriate time
to start the listener thread, hence exposing an API for higher
layer code to explicitly start listeners.
Test Plan: Imported from OSS
Differential Revision: D17932271
Pulled By: mrshenli
fbshipit-source-id: 3b408477594d4d19319e7cd08dd6f383a7ed7670
