Returns problem_hparams.

    Args:
      defaults: default hyperparameters
      unused_model_hparams: model hyperparameters

def hparams(self, defaults, unused_model_hparams):
    """Returns problem_hparams.

    Args:
      defaults: default hyperparameters
      unused_model_hparams: model hyperparameters

    """
    (super(BabiQa, self).hparams(defaults, unused_model_hparams))
    p = defaults
    num_classes = self._encoders["targets"].vocab_size
    p.modality = {"targets": modalities.ModalityType.CLASS_LABEL}
    p.vocab_size = {"targets": num_classes}