Fix Kfold iterator

The `Kfold` iterator was returning the validation set indexes instead of the training set indexes, which was inconsistent with the other `CrossValGenerator`. 

Commit 60de1f1 fixed the `cross_validate` function for the broken `Kfold` iterator but broke it for the other `CrossValGenerators`. 

This pull request fixes the `Kfold` iterator and reverts 60de1f1.