[Distributed] Adding getNumKeys support to the HashStore (#46048)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46048
This PR adds support for the getNumKeys API for the HashStore
ghstack-source-id: 113874241
Test Plan: Added C++ tests for the HashStore::getNumKeys
Reviewed By: jiayisuse
Differential Revision: D24067658
fbshipit-source-id: 2db70a90f0ab8ddf0ff03cedda59b45ec987af07
