Make Optuna compatible with hold out set validation

[thomasATbayer]

The callback logic has to be improved to automatically skip Optuna early stopping when hold-out validation or any cross-validation with fewer than 2 splits is used, preventing errors and ensuring correct behavior. This allows the use of hold out sets.

[thomasATbayer]

 if self.early_stopping_optuna:
        # Check for hold-out (single fold) if cross_validation is provided
        if cross_validation is not None:
            n_splits = cross_validation.get_n_splits()
            if n_splits < 2:
                module_logger.warning(
                    "Optuna early stopping requires at least 2 CV folds. "
                    "Skipping early stopping (hold-out detected, n_splits=%s)",
                    n_splits,
                )
                return None