[GraphBolt] Modify `from_dglgraph()` (#6470)

python/dgl/graphbolt/impl/csc_sampling_graph.py

@@ -825,7 +825,12 @@ def from_dglgraph(
     include_original_edge_id: bool = False,
 ) -> CSCSamplingGraph:
     """Convert a DGLGraph to CSCSamplingGraph."""
+
     homo_g, ntype_count, _ = to_homogeneous(g, return_count=True)
+
+    if is_homogeneous:
+        metadata = None
+    else:
         # Initialize metadata.
         node_type_to_id = {ntype: g.get_ntype_id(ntype) for ntype in g.ntypes}
         edge_type_to_id = {

tests/python/pytorch/graphbolt/impl/test_csc_sampling_graph.py

@@ -1321,8 +1321,7 @@ def test_from_dglgraph_homogeneous():
     assert gb_g.total_num_edges == dgl_g.num_edges()
     assert torch.equal(gb_g.node_type_offset, torch.tensor([0, 1000]))
     assert gb_g.type_per_edge is None
-    assert gb_g.metadata.node_type_to_id == {"_N": 0}
-    assert gb_g.metadata.edge_type_to_id == {"_N:_E:_N": 0}
+    assert gb_g.metadata is None
 
 
 @unittest.skipIf(