add docs to onnx APIs (#22938)
Summary:
Add docs to onnx APIs, including
- export
- export_to_pretty_string
- is_in_onnx_export
Fix https://github.com/pytorch/pytorch/issues/14698
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22938
Differential Revision: D16296182
Pulled By: zhangguanheng66
fbshipit-source-id: 1a1fa769b430db6428e6dfafba5447e6e2a75517
