Support SoftmaxCrossEntropyLoss

[hgaspar]

This is a very common ONNX operator.

ORT falls back to CPU when encountering this node. The code is at:

https://github.com/microsoft/onnxruntime/blob/ee603ee3265dbf6eac112baf273b6b69bf696085/onnxruntime/core/providers/migraphx/migraphx_execution_provider.cc#L1022-L1029

This happens when executing LLama v2, from:

https://github.com/microsoft/onnxruntime-inference-examples/blob/main/python/models/llama/LLaMA-2 E2E Notebook.ipynb

[TedThemistokleous]

Relevant doc/spec - https://github.com/onnx/onnx/blob/main/docs/Operators.md#softmaxcrossentropyloss