7. Multi-label data stratification
#scikit-multilearn
http://scikit.ml/stratification.html
iterative_train_test_split
skmultilearn.model_selection.iterative_train_test_split
（積ん読）On the Stratification of Multi-Label Data
別パッケージ（
#iterative-stratification
）でも：
multilabelでStratifiedKFold