[ONNX] Update ONNX documentation (#51362) (#53313)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53313
Add information about .data field
Test Plan: Imported from OSS
Reviewed By: pbelevich, malfet
Differential Revision: D26922421
Pulled By: SplitInfinity
fbshipit-source-id: 5117ac20990e286dcacb44f7b810b1bcc75d3dd6
