private void process(ExpressionNode node, RankProfile profile) {
        if (node instanceof ReferenceNode) {
            process((ReferenceNode)node, profile);
        } else if (node instanceof CompositeNode) {
            for (ExpressionNode child : ((CompositeNode) node).children()) {
                process(child, profile);
            }
        }
    }
Processes ONNX ranking features of the form:

    onnx("files/model.onnx", "path/to/output:1")

And generates an "onnx-model" configuration as if it was defined in the profile:

  onnx-model files_model_onnx {
      file:  "files/model.onnx"
  }

Inputs and outputs are resolved in OnnxModelTypeResolver, which must be
processed after this.

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