[Test] Add testcase for function AddSelfLoop (#5858)

Co-authored-by: Ubuntu <ubuntu@ip-172-31-6-31.ap-northeast-1.compute.internal>

tests/python/common/transforms/test_transform.py

@@ -2427,7 +2427,7 @@ def test_module_add_self_loop(idtype):
     assert "h" in new_g.ndata
     assert "w" in new_g.edata
 
-    # Case2: Remove self-loops first to avoid duplicate ones
+    # Case2: remove self-loops first to avoid duplicate ones
     transform = dgl.AddSelfLoop(allow_duplicate=True)
     new_g = transform(g)
     assert new_g.device == g.device
@@ -2440,6 +2440,17 @@ def test_module_add_self_loop(idtype):
     assert "h" in new_g.ndata
     assert "w" in new_g.edata
 
+    # Case3: add self-loops for a homogeneous graph (the example in doc)
+    transform = dgl.AddSelfLoop(fill_data="sum")
+    g = dgl.graph(([0, 0, 2], [2, 1, 0]), idtype=idtype, device=F.ctx())
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.num_nodes() == g.num_nodes()
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 2), (0, 1), (2, 0), (0, 0), (1, 1), (2, 2)}
+
     # Create a heterogeneous graph
     g = dgl.heterograph(
         {
@@ -2454,7 +2465,7 @@ def test_module_add_self_loop(idtype):
     g.nodes["game"].data["h2"] = F.randn((2, 4))
     g.edges["follows"].data["w2"] = F.randn((1, 5))
 
-    # Case3: add self-loops for a heterogeneous graph
+    # Case4: add self-loops for a heterogeneous graph
     new_g = transform(g)
     assert new_g.device == g.device
     assert new_g.idtype == g.idtype
@@ -2469,7 +2480,7 @@ def test_module_add_self_loop(idtype):
     assert "w1" in new_g.edges["plays"].data
     assert "w2" in new_g.edges["follows"].data
 
-    # Case4: add self-etypes for a heterogeneous graph
+    # Case5: add self-etypes for a heterogeneous graph
     transform = dgl.AddSelfLoop(new_etypes=True)
     new_g = transform(g)
     assert new_g.device == g.device