move get_rpc_timeout to pybind (#29765)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29765
instead of wrapping this C++ function with python that causes
unnecessary overhead, we can move this to pybind and use the `DefaultRpcAgent`
to get the timeout.
ghstack-source-id: 93879236
Test Plan: unit tests pass
Differential Revision: D18493195
fbshipit-source-id: fd0f1f13ee15acb5ea1ae7c696925c9b54304f6d
