Expand split ops (#2505)

* candle-onnx: Add Split and Expand operators, Fix Where Op

Implemented based on https://github.com/onnx/onnx/blob/main/docs/Operators.md
Test cases based on those examples.

TODO: Should add the remaining Split examples as tests
TODO: Add.test case that motivates Where fix

* candle-onnx: Add ReduceSum operator

Implemented based on https://github.com/onnx/onnx/blob/main/docs/Operators.md
Test cases based on those examples.

TODO: Should add the remaining ReduceSum examples as tests

* candle-onnx: Add ReduceL2 operator

Implemented based on https://github.com/onnx/onnx/blob/main/docs/Operators.md
Test cases based on those examples.

TODO: Should add the remaining ReduceSum examples as tests

* candle-onnx: Fix Clip operator empty string as default arg issue

Optional input args may be signified by an empty string. The length of the input array is not enough because non optional args may follow optional ones.

I encountered this when trying to use the ONNX model found at https://huggingface.co/sentence-transformers/all-MiniLM-L6-v2 for example.

The LSTM op has a utility which I factored to be more generally accessible, and I have used it in the ops I have recently created or debugged.

I believe it is likely that this issue may also manifest in other ops, but I didn't want to change anything that I'm not testing.

* fix formatting

* fix small mistake made during refactor

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