Fix dead code elimination in onnx export (#22476)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22476
Dead code elimination assumes a valid jit graph because it checks if operators have side effects.
The onnx export path destroys the jit graph right before calling dead code elimination, but it actually doesn't care about side effects.
We can just call dead code elimination and disable side effect lookup and things should work.
Reviewed By: houseroad
Differential Revision: D16100172
fbshipit-source-id: 8c790055e0d76c4227394cafa93b07d1310f2cea
