[python] add parameter object_hook to method dump_model

[xadupre]

Method dump_model parses a string to create a JSON structure. If a function needs to modify this structure, it needs to walk through the whole structure a second time. The object_hook can be used to do both in one pass. This trick was used to significantly speed up the conversion of an heavy model (>40.000 trees) into ONNX (onnx/onnxmltools#491). This PR would avoid duplication of the code of dump_model in onnxmltools.

[StrikerRUS]

@xadupre Thank you very much for the contribution!
Very interesting addition! TBH, I've never used this argument of json.loads().
Impressive results in the ONNX speedup! I believe new argument might be useful for other users and it's worth exposing object_hook in the dump_model() signature.

Generally LGTM, just one comment about numpy doc style below.

[jameslamb]

Thanks very much for this! Really nice contribution and we appreciate you taking the time to upstream it into LightGBM.

I agree with @StrikerRUS , this is a great addition and just needs a slight documentation update

[xadupre]

Thanks for the feedback. I improved the documentation.

[StrikerRUS]

Thanks a lot! LGTM, except one minor nit about emphasizing style in the docstring.

Ah, forgot to say special thanks for adding test! 🙂

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