directional GRU with pretrained word vectors
                      - &#39;bert&#39; for BERT Text Classification
                      - &#39;distilbert&#39; for Hugging Face DistilBert model

        train_data (tuple): a tuple of numpy.ndarrays: (x_train, y_train) or ktrain.Dataset instance
                            returned from one of the texts_from_* functions
        preproc: a ktrain.text.TextPreprocessor instance.
                 As of v0.8.0, this is required.
        multilabel (bool):  If True