[GraphBolt] Correct `to_pyg_data` (#7124)

python/dgl/graphbolt/minibatch.py

@@ -500,7 +500,7 @@ class MiniBatch:
             col_nodes = torch.cat(col_nodes)
             row_nodes = torch.cat(row_nodes)
             edge_index = torch.unique(
-                torch.stack((col_nodes, row_nodes)), dim=1
+                torch.stack((row_nodes, col_nodes)), dim=1
             )
 
         if self.node_features is None:

tests/python/pytorch/graphbolt/test_minibatch.py

@@ -881,7 +881,7 @@ def test_to_pyg_data():
         original_column_node_ids=torch.tensor([10, 11]),
     )
     expected_edge_index = torch.tensor(
-        [[0, 0, 1, 1, 1, 2, 2, 3], [0, 1, 0, 1, 2, 1, 2, 2]]
+        [[0, 0, 1, 1, 1, 2, 2, 2], [0, 1, 0, 1, 2, 1, 2, 3]]
     )
     expected_node_features = torch.tensor([[1], [2], [3], [4]])
     expected_labels = torch.tensor([0, 1])