ENH reuse parent histogram

[lorentzenchr]

Summary

If a node is split of feature "a", then the 2 histograms of the child nodes for that feature "a" are trivial, just the left/right part of the parent's histogram.

Motivation

In particular for settings with only a small number of features, this can be beneficial for fitting time.

Description

References

scikit-learn/scikit-learn#26189

Notes

I'm not 100% sure, if this is already done in LightGBM or not.