Return the graph definition, if there is one.

    If the graph is stored directly, return that.  If no graph is stored
    directly but a metagraph is stored containing a graph, return that.

    Raises:
      ValueError: If there is no graph for this run.

    Returns:
      The `graph_def` proto.

def Graph(self):
    """Return the graph definition, if there is one.

    If the graph is stored directly, return that.  If no graph is stored
    directly but a metagraph is stored containing a graph, return that.

    Raises:
      ValueError: If there is no graph for this run.

    Returns:
      The `graph_def` proto.
    """
    graph = graph_pb2.GraphDef()
    if self._graph is not None:
      graph.ParseFromString(self._graph)
      return graph
    raise ValueError('There is no graph in this EventAccumulator')