public static String getModelConfigName(Reference reference) {
        if (reference.arguments().size() > 0) {
            ExpressionNode expr = reference.arguments().expressions().get(0);
            if (expr instanceof ReferenceNode) {  // refers to onnx-model config
                return expr.toString();
            }
            if (expr instanceof ConstantNode) {  // refers to a file path
                return asValidIdentifier(expr);
            }
        }
        return null;
    }
Transforms ONNX model features of the forms:

    onnx(config_name)
    onnx(config_name).output
    onnx("path/to/model")
    onnx("path/to/model").output
    onnx("path/to/model", "path/to/output")
    onnx("path/to/model", "unused", "path/to/output")    // signature is unused

To the format expected by the backend:

    onnx(config_name).output

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