fixed pgl for pslib; fixed example in citation_network

pgl/graph_wrapper.py

@@ -836,11 +836,12 @@ class BatchGraphWrapper(BaseGraphWrapper):
     def __build_edges(self, edges, node_shift, edge_lod):
         """ Merge subgraph edges. 
         """
-        if len(edges) == 2:
+        if isinstance(edges, tuple):
             src, dst  = edges
         else:
             src = edges[:, 0]
             dst = edges[:, 1]
+
         src = L.reshape(src, [-1])
         dst = L.reshape(dst, [-1])
         src = paddle_helper.ensure_dtype(src, dtype="int32")