[ONNX] Update ONNX doc for indexing export (#46349)
Summary:
Adding example code for supported cases.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46349
Reviewed By: gchanan
Differential Revision: D24459449
Pulled By: malfet
fbshipit-source-id: 65021a96cd12225615aa40af5d916e0cda56d107
