[ONNX] Update pytorch/onnx docs for new export API args (#39802)
Summary:
Update pytorch/onnx docs for new export API args:
Use external data format and Training args.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39802
Reviewed By: hl475
Differential Revision: D22139664
Pulled By: houseroad
fbshipit-source-id: 7d6dcf75129cb88987f8c37b7d9d48ca594c0f38
