Set onnx opset version before model select (#37466)
Summary:
Set opset version before model select call - which is used to trigger warnings.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37466
Reviewed By: hl475
Differential Revision: D21308796
Pulled By: houseroad
fbshipit-source-id: 0974b9d5b6562d4451f54053138174f663a17aa3
