Column name as a string of the outcome variable
(default NULL; the first column will be chosen automatically).
perf_metric_function
Function to calculate the performance metric to
be used for cross-validation and test performance. Some functions are
provided by caret (see caret::defaultSummary()).
Defaults: binary classification = twoClassSummary,
multi-class classification = multiClassSummary,
regression = defaultSummary.
class_probs
Whether to use class probabilities (TRUE for categorical outcomes, FALSE for numeric outcomes).