[Bugfix] `dgl.linkx_homophily` calculation fails when all class homophily=0 (#5720)

Co-authored-by: Mufei Li <mufeili1996@gmail.com>

python/dgl/homophily.py

@@ -177,7 +177,7 @@ def linkx_homophily(graph, y):
         num_nodes = graph.num_nodes()
         num_classes = y.max(dim=0).values.item() + 1
 
-        value = 0
+        value = torch.tensor(0.0).to(graph.device)
         for k in range(num_classes):
             # Get the nodes that belong to class k.
             class_mask = y == k

tests/python/common/test_homophily.py

@@ -48,3 +48,6 @@ def test_linkx_homophily(idtype):
     graph = dgl.graph(([0, 1, 2, 3], [1, 2, 0, 4]), device=device)
     y = F.tensor([0, 0, 0, 0, 1])
     assert math.isclose(dgl.linkx_homophily(graph, y), 0.19999998807907104)
+
+    y = F.tensor([0, 1, 2, 3, 4])
+    assert math.isclose(dgl.linkx_homophily(graph, y), 0.0000000000000000)