def add_output(self, output_proto: ValueInfoProto, graph_idx: int):
        """Add an output to the graph.

        :param output_proto: ValueInfoProto of the output.
        :param graph_idx: index of the graph in the model.
        """
        if output_proto.name in self.ios:
            raise ValueError(f"Output {output_proto.name} already exists in the graph.")
        self.ios[output_proto.name] = OnnxIO(
            proto=[output_proto], destination=[SpecialOutput.OUTPUT], graph_idx=graph_idx
        )
Add an output to the graph.

        :param output_proto: ValueInfoProto of the output.
        :param graph_idx: index of the graph in the model.
        
olive/passes/onnx/onnx_dag.py