@Override
    public ExpressionNode transform(ExpressionNode node, RankProfileTransformContext context) {
        if (node instanceof ReferenceNode)
            return transformFeature((ReferenceNode) node, context);
        else if (node instanceof CompositeNode)
            return super.transformChildren((CompositeNode) node, context);
        else
            return node;
    }
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