ONNX: Adds Ops GatherElements & Xor

[AnubhabB]

This PR adds support for the following ONNX Ops:

• GatherElements - closes ONNX: unsupported GatherElements ops #2465
• Xor

@LaurentMazare please take a look at the Xor implementation, Candle is currently missing Tensor::logical_<and | or | not | xor> ops?

Unsure if my current workaround is reasonable.

[LaurentMazare]

The xor implementation looks reasonable to me in the current setup. I would be curious if you have any practical model that require this op?

[AnubhabB]

Yup, this one - this is a Document layout analysis model I crossed path with by Unstructured.io, originally I had intended to create a quick example for candle-onnx but refrained because I can't find the license information associated with the model.

[AnubhabB]

In fact, these are the ops used by this model:

["Add", "And", "Cast", "Clip", "Concat", "Constant", "ConstantOfShape", "Conv", "ConvTranspose", "Div", "Equal", "Exp", "Expand", "Flatten", "Floor", "Gather", "GatherND", "Gemm", "Greater", "If", "Less", "Log", "MaxPool", "Mod", "Mul", "NonZero", "Not", "Pad", "Range", "ReduceMax", "ReduceProd", "Relu", "Reshape", "Resize", "RoiAlign", "ScatterND", "Shape", "Sigmoid", "Slice", "Softmax", "Split", "Sqrt", "Squeeze", "Sub", "TopK", "Transpose", "Unsqueeze", "Where", "Xor"]

I'll try and figure out if these ops are common across some of the more popular models in Detectron2 Model Zoo. If they are common I'll try and bridge the gaps with Candle implementation over time, otherwise I think I'm better off with something like Sonos/tract or ort based onnx runtimes for my current project!

Edit: From the Ops above - the following are not supported by candle-onnx at the moment:

["And", "ConvTranspose", "GatherND", "Mod", "NonZero", "ReduceProd", "Resize", "RoiAlign", "ScatterND", "TopK"]

From what I can tell detectron2 mask rcnn based models exported to onnx tend to use these ops.
I'll probably take a stab at implementing these.