[PyTorch Mobile] Move string op registrations to prim and make them selective (#44960)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44960
Since we have templated selective build, it should be safe to move the operators to prim so that they can be selectively built in mobile
Test Plan: CI
Reviewed By: linbinyu
Differential Revision: D23772025
fbshipit-source-id: 52cebae76e4df5a6b2b51f2cd82f06f75e2e45d0
