Make static dispatch turn off variable before entering the kernel (#28398)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28398
Redo PR #26908 for issue #26764
Test Plan: - make sure quantized mobilenetv2 no longer suffers from perf regression
Differential Revision: D18055851
Pulled By: ljk53
fbshipit-source-id: d533bc8979b1d2892adfb39924678a3f9b591855
