[C2] Add string equality operator (#45886)
Summary:
This diff adds a string equality checking operator.
Another attempt at reverted D24042344 (https://github.com/pytorch/pytorch/commit/cf48872d28f945d47793f63e19c54dd15bf580f7)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45886
Test Plan: unit tests, github builds
Reviewed By: dzhulgakov
Differential Revision: D24129953
fbshipit-source-id: caa53c7eac5c67c414c37e9d93416104f72556b9
