H. Masui

@OmarManzoor Thank you for more comments. I applied your suggestions to the test code. > Also I think we are still not handling the case where if an estimator/regressor that...

Note that `StackingClassifier` is already available for multilabel classification problem but unavailable for multiclass-multioutput classification. I think that the latter is an issue that is out of scope this PR....

@baskaryan @efriis @eyurtsev @ccurme @vbarda @hwchase17 Could anyone review this PR?

Thanks for fix @ZhangShenao > Would love to get @hmasdev or @eyurtsev eyes to make sure this wasn't intentional on #19792 @efriis this PR looks correct to me. 'input' keyword...