Set the default ONNX opset to the latest stable opset (i.e., 9) (#17736)
Summary:
1) The changes in the new opset won't affect internal pipeline.
2) The CI won't be affected by the ONNX changes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17736
Reviewed By: zrphercule
Differential Revision: D14358710
Pulled By: houseroad
fbshipit-source-id: 4ef15d2246b50f6875ee215ce37ecf92d555ca6a
