ONNX: unsupported GatherElements ops

[hdtrinh]

Currently GatherElements operator is not supported in candle-onnx. This would be convenient since a number of converted models using HummingBird ML use this operator (including Tree Classifiers).

GatherElements slightly differs from Gather which is already supported. GatherElements is already supported by burn. Defined here: https://onnx.ai/onnx/operators/onnx__GatherElements.html