Return a json dictionary representing this model.

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