[DistGB] fix prob attr not exist (dmlc#7783)

Author: Rhett-Ying

python/dgl/distributed/dist_dataloader.py

@@ -378,7 +378,10 @@ def __init__(self, g, nids, graph_sampler):
         self._dataset = utils.maybe_flatten_dict(self.nids)
 
         # Add prob/mask into graphbolt partition's edge attributes if needed.
-        Collator.add_edge_attribute_to_graph(self.g, self.graph_sampler.prob)
+        if hasattr(self.graph_sampler, "prob"):
+            Collator.add_edge_attribute_to_graph(
+                self.g, self.graph_sampler.prob
+            )
 
     @property
     def dataset(self):
@@ -637,7 +640,10 @@ def __init__(
         self._dataset = utils.maybe_flatten_dict(self.eids)
 
         # Add prob/mask into graphbolt partition's edge attributes if needed.
-        Collator.add_edge_attribute_to_graph(self.g, self.graph_sampler.prob)
+        if hasattr(self.graph_sampler, "prob"):
+            Collator.add_edge_attribute_to_graph(
+                self.g, self.graph_sampler.prob
+            )
 
     @property
     def dataset(self):