Is there a small example how to use Dagre.NET to do Nested graph layout?  [Bert Onnx ](https://github.com/onnx/models/blob/master/text/machine_comprehension/bert-squad/model/bertsquad-10.onnx)has a higher level Nested layout e.g. NX involving encoding and deconding. We may need a discussion on how to best present the Transformer graph layout 