private static void importSignatures(IntermediateGraph graph, ImportedModel model) {
        for (String signatureName : graph.signatures()) {
            ImportedModel.Signature signature = model.signature(signatureName);
            for (Map.Entry<String, String> input : graph.inputs(signatureName).entrySet()) {
                signature.input(input.getKey(), input.getValue());
            }
            for (Map.Entry<String, String> output : graph.outputs(signatureName).entrySet()) {
                signature.output(IntermediateOperation.vespaName(output.getKey()), output.getValue());
            }
        }
    }
Takes an IntermediateGraph and converts it to a ImportedModel containing
the actual Vespa ranking expressions.
model-integration/src/main/java/ai/vespa/rankingexpression/importer/ModelImporter.java