keras/src/legacy/preprocessing/image.py/DataFrameIterator/get_classes
class DataFrameIterator:    def get_classes(self, df, y_col):
        labels = []
        for label in df[y_col]:
            if isinstance(label, (list, tuple)):
                labels.append([self.class_indices[lbl] for lbl in label])
            else:
                labels.append(self.class_indices[label])
        return labels