We have no clear documentation on how to handle supervised training.....

... while using vanilla-biometrics.

Internally, we know that we need to add this kwarg to the transformer that does the fit fit_extra_arguments = (("y", "subject_id"),) linking the subject_id from the sample with the y parameter of the BaseEstimator. However, this is obscure in the documentation.

We need a proper documentation and a simple example.