Construct symbol from onnx graph.

        Parameters
        ----------
        graph : onnx protobuf object
            The loaded onnx graph

        Returns
        -------
        sym :symbol.Symbol
            The returned mxnet symbol
        params : dict
            A dict of name: nd.array pairs, used as pretrained weights

def from_onnx(self, graph):
        """Construct symbol from onnx graph.

        Parameters
        ----------
        graph : onnx protobuf object
            The loaded onnx graph

        Returns
        -------
        sym :symbol.Symbol
            The returned mxnet symbol
        params : dict
            A dict of name: nd.array pairs, used as pretrained weights
        """
        # get input, output shapes
        self.model_metadata = self.get_graph_metadata(graph)
        # parse network inputs, aka parameters
        for init_tensor in graph.initializer:
            if not init_tensor.name.strip():
                raise ValueError("Tensor's name is required.")
            self._params[init_tensor.name] = self._parse_array(init_tensor)

        # converting GraphProto message
        for i in graph.input:
            if i.name in self._params:
                # i is a param instead of input
                self._nodes[i.name] = symbol.Variable(name=i.name,
                                                      shape=self._params[i.name].shape)
            else:
                self._nodes[i.name] = symbol.Variable(name=i.name)

        # constructing nodes, nodes are stored as directed acyclic graph
        # converting NodeProto message
        for node in graph.node:
            op_name = node.op_type
            node_name = node.name.strip()
            node_name = node_name if node_name else None
            onnx_attr = self._parse_attr(node.attribute)
            inputs = [self._nodes[i] for i in node.input]
            mxnet_sym = self._convert_operator(node_name, op_name, onnx_attr, inputs)

            for k, i in zip(list(node.output), range(len(mxnet_sym.list_outputs()))):
                self._nodes[k] = mxnet_sym[i]

            # splitting params into args and aux params
            for args in mxnet_sym.list_arguments():
                if args in self._params:
                    self.arg_dict.update({args: nd.array(self._params[args])})
            for aux in mxnet_sym.list_auxiliary_states():
                if aux in self._params:
                    self.aux_dict.update({aux: nd.array(self._params[aux])})

        # now return the outputs
        out = [self._nodes[i.name] for i in graph.output]
        if len(out) > 1:
            out = symbol.Group(out)
        else:
            out = out[0]
        return out, self.arg_dict, self.aux_dict