Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'lemma') and self.lemma is not None:
            _dict['lemma'] = self.lemma
        if hasattr(self, 'part_of_speech') and self.part_of_speech is not None:
            _dict['part_of_speech'] = self.part_of_speech
        return _dict