private static int getOutputIndex(Onnx.NodeProto node, String outputName) {
        return node.getOutputCount() == 0 ? 0 : Math.max(node.getOutputList().indexOf(outputName), 0);
    }
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