[ONNX] Update embedding export wrt padding_idx (#53931)
Summary:
To be in-sync with https://github.com/pytorch/pytorch/issues/53447
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53931
Reviewed By: ngimel
Differential Revision: D27026616
Pulled By: malfet
fbshipit-source-id: 4c50b29fa296c90aeeeb1757bdaada92cbba33d4
