private ExpressionNode transformFeature(ReferenceNode feature, RankProfileTransformContext context) {
        if (context.rankProfile() == null) return feature;
        if (context.rankProfile().schema() == null) return feature;
        return transformFeature(feature, context.rankProfile());
    }
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