add the support for stable ONNX opsets in exporter (#16068)
Summary:
Still wip, need more tests and correct handling for opset 8 in symbolics.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16068
Reviewed By: zrphercule
Differential Revision: D14185855
Pulled By: houseroad
fbshipit-source-id: 55200be810c88317c6e80a46bdbeb22e0b6e5f9e
