[Graphbolt] Add fanout check (#5776)

python/dgl/graphbolt/graph_storage/csc_sampling_graph.py

@@ -269,6 +269,10 @@ class CSCSamplingGraph:
             (fanouts >= 0) | (fanouts == -1)
         ), "Fanouts should consist of values that are either -1 or \
             greater than or equal to 0."
+        if self.metadata and self.metadata.edge_type_to_id:
+            assert len(self.metadata.edge_type_to_id) == fanouts.size(
+                0
+            ), "Fanouts should have the same number of elements as etypes."
         return self._c_csc_graph.sample_neighbors(
             nodes, fanouts.tolist(), replace, return_eids
         )