[ONNX] Update ONNX doc for writing pytorch model (#46961)
Summary:
For tracing successfully, we need write pytorch model in torch way. So we add instructions with examples here.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46961
Reviewed By: ailzhang
Differential Revision: D24900040
Pulled By: bzinodev
fbshipit-source-id: b375b533396b11dbc9656fa61e84a3f92f352e4b
