Add dropout removal pass. (#38253)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38253
This pass removes dropout and dropout_ nodes when training is false. It
requires to have run freeze_module pass which does both inlining and constant
propagation, without which training variable remains as attribute instead of
constant.
ghstack-source-id: 103939141
Test Plan: python test/test_jit.py TestScript.test_remove_dropout
Reviewed By: dreiss
Differential Revision: D21505863
fbshipit-source-id: 42ea45804e4653b625b6a254c8d8480757264aa8
