[ONNX] Add support for scalar src in torch.scatter ONNX export. (#42765)
Summary:
`torch.scatter` supports two overloads – one where `src` input tensor is same size as the `index` tensor input, and second, where `src` is a scalar. Currrently, ONNX exporter only supports the first overload. This PR adds export support for the second overload of `torch.scatter`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42765
Reviewed By: hl475
Differential Revision: D23025189
Pulled By: houseroad
fbshipit-source-id: 5c2a3f3ce3b2d69661a227df8a8e0ed7c1858dbf
