resenting classes (class_names is required)
                       4. numerical values for text regresssion (class_names should be left empty)
        class_names (list): list of strings representing class labels
                            shape should be (num_examples,1) or (num_examples,)
        max_features(int): max num of words to consider in vocabulary
                           Note: This is only used for preprocess_mode=&#39;standard&#39;.
        maxlen(int): each document can be of most &lt;maxl