Return whether the graph is a uni-bipartite graph.
A uni-bipartite heterograph can further divide its node types into two sets: SRC and DST. All edges are from nodes in SRC to nodes in DST. The following APIs can be used to get the type, data, and nodes that belong to SRC and DST sets:
Note that we allow two node types to have the same name as long as one belongs to SRC while the other belongs to DST. To distinguish them, prepend the name with
"DST/"when specifying a node type.