private static String getNodeName(Onnx.NodeProto node) {
        String nodeName = node.getName();
        if (nodeName.length() > 0)
            return nodeName;
        if (node.getOutputCount() == 1)
            return node.getOutput(0);
        throw new IllegalArgumentException("Unable to find a suitable name for node '" + node.toString() + "'. " +
                "Either no explicit name given or no single output name.");
    }
Converts an ONNX graph to a Vespa IntermediateGraph which is the basis
for generating Vespa ranking expressions.

@author lesters
model-integration/src/main/java/ai/vespa/rankingexpression/importer/onnx/GraphImporter.java