Return the metagraph definition, if there is one.

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

    Returns:
      The `meta_graph_def` proto.

def MetaGraph(self):
    """Return the metagraph definition, if there is one.

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

    Returns:
      The `meta_graph_def` proto.
    """
    if self._meta_graph is None:
      raise ValueError('There is no metagraph in this EventAccumulator')
    meta_graph = meta_graph_pb2.MetaGraphDef()
    meta_graph.ParseFromString(self._meta_graph)
    return meta_graph