add import for example of graphdata

mindspore/dataset/engine/graphdata.py

@@ -302,6 +302,7 @@ class GraphData:
             The first one is offset table, the second one is neighbors
 
         Examples:
+            >>> from mindspore.dataset.engine import OutputFormat
             >>> nodes = graph_dataset.get_all_nodes(node_type=1)
             >>> neighbors = graph_dataset.get_all_neighbors(node_list=nodes, neighbor_type=2)
             >>> neighbors_coo = graph_dataset.get_all_neighbors(node_list=nodes, neighbor_type=2,