private static void getWarnings(IntermediateOperation op, java.util.Map<String, Set<String>> warnings) {
        if (warnings.containsKey(op.name())) return;
        op.inputs().forEach(input -> getWarnings(input, warnings));
        warnings.put(op.name(), new HashSet<>(op.warnings()));
    }
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