Add syntax 'with average_model.apply(exe)'

fluid/ocr_recognition/ctc_train.py

@@ -81,8 +81,7 @@ def train(args, data_reader=dummy_reader):
                 sys.stdout.flush()
             batch_id += 1
 
-        if model_average != None:
-            model_average.apply(exe)
+        with model_average.apply(exe):
             error_evaluator.reset(exe)
             for data in test_reader():
                 exe.run(inference_program, feed=get_feeder_data(data, place))