Return a json dictionary representing this model.

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