Suppressing warnings generated by the `test_prop_nodes_topo` test. (#6651)

Co-authored-by: Hongzhi (Steve), Chen <chenhongzhi.nkcs@gmail.com>

tests/python/common/test_propagate.py

@@ -100,6 +100,14 @@ def test_prop_nodes_topo(idtype):
     tree.ndata["x"] = F.zeros((5, 2))
     # set all leaf nodes to be ones
     tree.nodes[[1, 3, 4]].data["x"] = F.ones((3, 2))
+
+    # Filtering DGLWarning:
+    #    The input graph for the user-defined edge
+    #    function does not contain valid edges
+    import warnings
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore", category=UserWarning)
         dgl.prop_nodes_topo(
             tree, message_func=mfunc, reduce_func=rfunc, apply_node_func=None
         )