class_docstr: A directed graph that can easily be JSON serialized for visualization.
  
  When serializing, it generates the following fields:
    edge: The list of all serialized Edge instances.
    node: The list of all serialized Vertex instances.
docstr: Returns or Creates a Vertex mapped by key.
  
  Args:
    key: A string reference for a vertex.  May refer to a new Vertex in which
    case it will be created.
  
  Returns:
    A the Vertex mapped to by key.
docstr_tok: ["Returns","or","Creates","a","Vertex","mapped","by","key",".","Args",":","key",":","A","string","reference","for","a","vertex",".","May","refer","to","a","new","Vertex","in","which","case","it","will","be","created",".","Returns",":","A","the","Vertex","mapped","to","by","key","."]
code: def get_vertex(self, key):
      """Returns or Creates a Vertex mapped by key.
  
      Args:
        key: A string reference for a vertex.  May refer to a new Vertex in which
        case it will be created.
  
      Returns:
        A the Vertex mapped to by key.
      """
      if key in self.vertex_map:
          return self.vertex_map[key]
      vertex = self.new_vertex()
      self.vertex_map[key] = vertex
      return vertex
  
file_path: tensor2tensor/tensor2tensor/insights/graph.py
label_desc: Use image node_id 3 for calling the Graph obj's underlying member method code with example usage: obj.get_vertex(key) and returns: vertex, self