[Bugfix] Update deprecated method name in load_graph.py (#2769)

Method `dataset.num_labels` has been deprecated and replaced by `dataset.num_classes`.  
Updating the method name to avoid runtime warning.
Co-authored-by: Minjie Wang <wmjlyjemaine@gmail.com>

examples/pytorch/graphsage/load_graph.py

@@ -9,7 +9,7 @@ def load_reddit():
     g = data[0]
     g.ndata['features'] = g.ndata['feat']
     g.ndata['labels'] = g.ndata['label']
-    return g, data.num_labels
+    return g, data.num_classes
 
 def load_ogb(name):
     from ogb.nodeproppred import DglNodePropPredDataset