3.2 Basic graph types基本的图类型
Graph – Undirected graphs with self loops（有自环的无向图）
DiGraph - Directed graphs with self loops（有自环的有向图）
MultiGraph - Undirected graphs with self loops and parallel edges（有自环的无向多重图）
MultiDiGraph - Directed graphs with self loops and parallel edges（有自环的有向多重图）