This allow to correctly handle mix of 1d array type when you use an axis argument see `r2_score` for instance.

For some metrics, the underlying implementation unfortunately requires a ravel to work correctly with various mix 1d-shape, e.g. see `matthews_corrcoef` or metrics using `_binary_clf_curve`.

But it is true that some of these have `ravel=True` and it is not necessary. I will correct this.