[ML] Assorted runtime optimisations for classification and regression #863
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The principal change is to bucket the candidate split points so we only check a subset of the splits when searching for the derivatives to update when computing split statistics. There are also some smaller optimisations which profiling showed up as worthwhile. The impact is largest for the case we have many metric valued features. These are pure runtime optimisations, i.e. the results are unaffected.