move USE_STATIC_DISPATCH from CI script to master cmake (#25696)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25696
Move the flag from CI to CMake so it's less magic and can be reused by
iOS build as well.
Test Plan: - will check CI
Differential Revision: D17202734
Pulled By: ljk53
fbshipit-source-id: da4f150cbcf2bb5624def386ce3699eff2a7446f
