docs: 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