[ONNX] Update in scatter ONNX export when scalar src has different type (#43440)
Summary:
`torch.scatter` allows `src` to be of different type when `src` is a scalar. This requires a an explicit cast op to be inserted in the ONNX graph because ONNX `ScatterElements` does not allow different types. This PR updates the export of `torch.scatter` with this logic.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43440
Reviewed By: hl475
Differential Revision: D23352317
Pulled By: houseroad
fbshipit-source-id: c9eeddeebb67fc3c40ad01def134799ef2b4dea6
