LightGBM: Support for outputting decision path and leaf

[ajryan]

Has any work already begun for outputting decision path and leaf data? I believe it would be similar to this implementation for decision tree and random forest in sklearn_onnx onnx/sklearn-onnx#563.

One challenge appears to be onnxmltools does not have a story for converters that accept options. I see that sklearn implemented their own topology and Scope that exposes an options API.

[ajryan]

Started work on this here: https://github.com/ajryan/onnxmltools/tree/aidanryan/feature/decision-path