class_docstr: Edge stores edge details connecting two Vertex instances.
  
  The Edge class supports serialization to a JSON data format expected by the
  client side representation.  When serializing, it generates the following
  fields:
    source_index: The source Vertex index for this Edge.
    target_index: The target Vertex index for this Edge.
    data: Arbitrary data for this Edge.
code: def __str__(self):
      return str(self.to_dict())
  
file_path: tensor2tensor/tensor2tensor/insights/graph.py
label_desc: Use image node_id 3 for calling the Edge obj's underlying member method code with example usage: obj.__str__() and returns: str