private static Set<String> getWarnings(IntermediateOperation op) {
        java.util.Map<String, Set<String>> warnings = new HashMap<>();
        getWarnings(op, warnings);
        return warnings.values().stream().flatMap(Collection::stream).collect(Collectors.toSet());
    }
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