[Distributed] getNumKeys API to c10d TCPStore (#43962)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43962
TCPStore needs a getNumKeys API for our logging needs.
ghstack-source-id: 112939761
Test Plan: Adding tests to C++ Store Tests
Reviewed By: pritamdamania87
Differential Revision: D22985085
fbshipit-source-id: 8a0d286fbd6fd314dcc997bae3aad0e62b51af83
