Pin onnx ir_version to 4 (#18768)
Summary:
to make test_operators.py more stable. in future, we will bump this up manually, and I think it's acceptable, since ir_version should be bumped too often.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18768
Reviewed By: zrphercule
Differential Revision: D14741514
Pulled By: houseroad
fbshipit-source-id: 0369dbc55424e345a113e49fc104a441ea290d58
