imporve the doc of decorate_reader API (#19206)

* imporve the doc of decorate_reader API, test=develop

* udpate API.spec, test=develop

paddle/fluid/API.spec

@@ -1017,9 +1017,9 @@ paddle.fluid.ParamAttr ('paddle.fluid.param_attr.ParamAttr', ('document', 'fa47f
 paddle.fluid.ParamAttr.__init__ (ArgSpec(args=['self', 'name', 'initializer', 'learning_rate', 'regularizer', 'trainable', 'gradient_clip', 'do_model_average'], varargs=None, keywords=None, defaults=(None, None, 1.0, None, True, None, False)), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
 paddle.fluid.WeightNormParamAttr ('paddle.fluid.param_attr.WeightNormParamAttr', ('document', 'b5ae1698ea72d5a9428000b916a67379'))
 paddle.fluid.WeightNormParamAttr.__init__ (ArgSpec(args=['self', 'dim', 'name', 'initializer', 'learning_rate', 'regularizer', 'trainable', 'gradient_clip', 'do_model_average'], varargs=None, keywords=None, defaults=(None, None, None, 1.0, None, True, None, False)), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
-paddle.fluid.DataFeeder ('paddle.fluid.data_feeder.DataFeeder', ('document', 'a39802654f20692ad49c340cef7c6556'))
+paddle.fluid.DataFeeder ('paddle.fluid.data_feeder.DataFeeder', ('document', 'd9e64be617bd5f49dbb08ac2bc8665e6'))
 paddle.fluid.DataFeeder.__init__ (ArgSpec(args=['self', 'feed_list', 'place', 'program'], varargs=None, keywords=None, defaults=(None,)), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
-paddle.fluid.DataFeeder.decorate_reader (ArgSpec(args=['self', 'reader', 'multi_devices', 'num_places', 'drop_last'], varargs=None, keywords=None, defaults=(None, True)), ('document', '449ec75d35b3498091908714e35e6686'))
+paddle.fluid.DataFeeder.decorate_reader (ArgSpec(args=['self', 'reader', 'multi_devices', 'num_places', 'drop_last'], varargs=None, keywords=None, defaults=(None, True)), ('document', 'a0ed5ce816b5d603cb595aacb922335a'))
 paddle.fluid.DataFeeder.feed (ArgSpec(args=['self', 'iterable'], varargs=None, keywords=None, defaults=None), ('document', 'ce65fe1d81dcd7067d5092a5667f35cc'))
 paddle.fluid.DataFeeder.feed_parallel (ArgSpec(args=['self', 'iterable', 'num_places'], varargs=None, keywords=None, defaults=(None,)), ('document', '334c6af750941a4397a2dd2ea8a4d76f'))
 paddle.fluid.clip.ErrorClipByValue ('paddle.fluid.clip.ErrorClipByValue', ('document', 'e6f815a03be88dee2537707d9e6b9209'))

python/paddle/fluid/data_feeder.py

@@ -171,7 +171,7 @@ class DataFeeder(object):
         
         feeder = fluid.DataFeeder(place=place, feed_list=[data, label])
         reader = feeder.decorate_reader(
-                paddle.batch(paddle.dataset.flowers.train(), batch_size=16), multi_devices=False)
+                paddle.batch(paddle.dataset.flowers.train(), batch_size=16), multi_devices=True)
 
     Args:
         feed_list(list): The Variables or Variables'name that will
@@ -395,22 +395,28 @@ class DataFeeder(object):
                 import numpy.random as random
                 import paddle
                 import paddle.fluid as fluid
+                import paddle.fluid.compiler as compiler
                 
-                def reader(limit=5):
+                def reader(limit=10):
                     for i in range(limit):
                         yield (random.random([784]).astype('float32'), random.random([1]).astype('int64')),
                 
-                place=fluid.CPUPlace()
+                place=fluid.CUDAPlace(0)
                 data = fluid.layers.data(name='data', shape=[1, 28, 28], dtype='float32')
                 label = fluid.layers.data(name='label', shape=[1], dtype='int64')
                 
+                hidden = fluid.layers.fc(input=data, size=10)
+                
                 feeder = fluid.DataFeeder(place=place, feed_list=[data, label])
-                reader = feeder.decorate_reader(reader, multi_devices=False)
+                reader = feeder.decorate_reader(reader, multi_devices=True)
                 
                 exe = fluid.Executor(place)
                 exe.run(fluid.default_startup_program())
-                for data in reader():
-                    exe.run(feed=data)
+                compiled_prog = compiler.CompiledProgram(
+                         fluid.default_main_program()).with_data_parallel()
+                for i,data in enumerate(reader()):
+                    print('iteration : ', i + 1)
+                    ret = exe.run(compiled_prog, feed=data, fetch_list=[hidden])
         """
 
         def __reader_creator__():