[ONNX] Cast Gather index to Long if needed (#47653)
Summary:
Onnx op Gather index need be int32 or int64. However, we don't have this Cast in our converter.
Therefore, it fails the following UT (for opset 11+)
`seq_length.type().scalarType()` is None, so `_arange_cast_helper()` cannot treat it as all integral, then it will cast all to float. Then this float value will be used as Gather index, hence it throws error in ORT about float type index.
The fix is that we need cast Gather index type to Long if it is not int/long.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47653
Reviewed By: heitorschueroff
Differential Revision: D25298056
Pulled By: mruberry
fbshipit-source-id: 05e3a70ccfd74612233c63ec5bb78e060b211909
