fix link prediction (#3485)

tutorials/blitz/4_link_predict.py

@@ -99,7 +99,7 @@ neg_u, neg_v = np.where(adj_neg != 0)
 
 neg_eids = np.random.choice(len(neg_u), g.number_of_edges())
 test_neg_u, test_neg_v = neg_u[neg_eids[:test_size]], neg_v[neg_eids[:test_size]]
-train_neg_u, train_neg_v = neg_u[neg_eids[train_size:]], neg_v[neg_eids[train_size:]]
+train_neg_u, train_neg_v = neg_u[neg_eids[test_size:]], neg_v[neg_eids[test_size:]]
 
 
 ######################################################################

tutorials/large/L2_large_link_prediction.py

@@ -395,7 +395,7 @@ test_neg_dst = torch.randint(0, graph.num_nodes(), (graph.num_edges(),))
 # You also need to label the edges, 1 if positive and 0 if negative.
 #
 
-test_src = torch.cat([test_pos_src, test_neg_src])
+test_src = torch.cat([test_pos_src, test_pos_dst])
 test_dst = torch.cat([test_neg_src, test_neg_dst])
 test_graph = dgl.graph((test_src, test_dst), num_nodes=graph.num_nodes())
 test_graph.edata['label'] = torch.cat(