LabelSegmentedKFold cross-validation iterator #4709
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This PR implements a variant of KFold that ensures that the train and test split is disjoint w.r.t. a third-party label. So far as I have seen all other Leave*Out methods either return all or a sample of all combinations of the third-party labels. However, if each party should be incorporated as test instance and the number of possible labels is huge, this k-fold variant is more efficient.
Example: Assume that we have 1.000.000 observations from 10.000 different users and we would like to estimate the generalization performance for new users. So, the train and test split should be disjoint, each user should occur once in the test set, and "leave one user out" would be intractable.
This is related to #4583.