Add pybind11 trampoline class for c10d.Store (#30415)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30415
This enables subclassing of c10d.Store and implementing its interface in Python.
ghstack-source-id: 94586627
Test Plan: New tests passes.
Reviewed By: vladbelous
Differential Revision: D18693018
fbshipit-source-id: fa1eba4bd11cc09a3d6bf3f35369c885033c63c0
