src/transformers/models/mbart/configuration_mbart.py/MBartOnnxConfig/outputs
class MBartOnnxConfig:    def outputs(self) -> Mapping[str, Mapping[int, str]]:
        if self.task in ["default", "seq2seq-lm"]:
            common_outputs = super().outputs
        else:
            common_outputs = super(OnnxConfigWithPast, self).outputs
            if self.use_past:
                num_encoder_layers, _ = self.num_layers
                for i in range(num_encoder_layers):
                    common_outputs[f"present.{i}.key"] = {0: "batch", 2: "past_sequence + sequence"}
                    common_outputs[f"present.{i}.value"] = {0: "batch", 2: "past_sequence + sequence"}
        return common_outputs