Another dce fix (#22499)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22499
Another place where onnx export is running dead code elimination after making the jit graph invalid. Fixing it.
Reviewed By: houseroad
Differential Revision: D16111969
fbshipit-source-id: 5ba80340c06d091988858077f142ea4e3da0638c
