code: public static <K> Graph<K, NullValue, NullValue> fromTuple2DataSet(DataSet<Tuple2<K, K>> edges,
  			ExecutionEnvironment context) {
  
  		DataSet<Edge<K, NullValue>> edgeDataSet = edges
  			.map(new Tuple2ToEdgeMap<>())
  				.name("To Edge");
  
  		return fromDataSet(edgeDataSet, context);
  	}
docs: Creates a graph from a DataSet of Tuple2 objects for edges.
  Each Tuple2 will become one Edge, where the source ID will be the first field of the Tuple2
  and the target ID will be the second field of the Tuple2.
  
  <p>Edge value types and Vertex values types will be set to NullValue.
  
  @param edges a DataSet of Tuple2.
  @param context the flink execution environment.
  @return the newly created graph.