[FileStore] Implemented numKeys and Added Tests (#49556)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49556
Implemented the missing Store functionality (specifically numKeys) in the FileStore.
Test Plan: Added both C++ and Python tests to verify functionality.
Reviewed By: jiayisuse
Differential Revision: D25619001
fbshipit-source-id: 9146d0da9e0903622be3035880f619bbb2cc3891
