fix python rpc handler exit crash (#27251)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27251
Explicitly clean up py::objects to avoid segment faults when py::objects with CPython are cleaned up later at program exit.
See similar issues reported https://github.com/pybind/pybind11/issues/1598
and https://github.com/pybind/pybind11/issues/1493.
Our local tests also caught this segment faults if py::objects are cleaned
up at program exit. The explaination is: CPython cleans up most critical
utitlies before cleaning up PythonRpcHandler singleton, so when
PythonRpcHandler signleton cleans up py::objects and call dec_ref(), it
will crash.
The solution is to clean up py::objects earlier when Rpc agent join().
Be note that py::objects can not be cleaned up when Rpc agent is destroyed
as well, as Rpc agent is global variable and it will have same issue as
PythonRpcHandler.
close #27182
ghstack-source-id: 92035069
Test Plan: unit tests on python 3.6 and python 3.5
Differential Revision: D17727362
fbshipit-source-id: c254023f6a85acce35528ba756a4efabba9a519f
