A Supervised model stores predictions for the training sample, and for the
validation and test samples when train received dat_validation and
dat_test. A SupervisedRes model stores the training and test
predictions of each outer resample. The training predictions are the model's
fitted values, stats::fitted(). A calibrated classifier also stores its
calibrated predictions, selected with calibrated = TRUE.
sample = NULL returns every stored sample, stacked in the order training,
validation, test. The samples are the same ones get_metrics() stores
metrics for. Requesting a sample the model does not store raises an error
that names the samples it does store.