private String validateInputSource(String source) {
        var optRef = Reference.simple(source);
        if (optRef.isPresent()) {
            Reference ref = optRef.get();
            // input can be one of:
            // attribute(foo), query(foo), constant(foo)
            if (FeatureNames.isSimpleFeature(ref)) {
                return ref.toString();
            }
            // or a function (evaluated by backend)
            if (ref.isSimpleRankingExpressionWrapper()) {
                var arg = ref.simpleArgument();
                if (arg.isPresent()) {
                    return ref.toString();
                }
            }
        } else {
            // otherwise it must be an identifier
            Reference ref = Reference.fromIdentifier(source);
            return ref.toString();
        }
        // invalid input source
        throw new IllegalArgumentException("invalid input for ONNX model " + getName() + ": " + source);
    }
A global ONNX model distributed using file distribution, similar to ranking constants.

@author lesters
config-model/src/main/java/com/yahoo/schema/OnnxModel.java