def output_chunk_length(self) -> Optional[int]:
        """Return `None` if none of the forecasting models have a `output_chunk_length`,
        otherwise return the smallest output_chunk_length.
        """
        tmp = [
            m.output_chunk_length
            for m in self.forecasting_models
            if m.output_chunk_length is not None
        ]

        if len(tmp) == 0:
            return None
        else:
            return min(tmp)
Return `None` if none of the forecasting models have a `output_chunk_length`,
        otherwise return the smallest output_chunk_length.
        
darts/models/forecasting/ensemble_model.py