fix layers. ==> fluid.layers. (#688)

Author: haowang101779990

doc/fluid/api_cn/fluid_cn.rst

@@ -1015,10 +1015,10 @@ feed map为该program提供输入数据。fetch_list提供program训练结束后
 ..  code-block:: python
 
 
-	data = layers.data(name='X', shape=[1], dtype='float32')
-	hidden = layers.fc(input=data, size=10)
+	data = fluid.layers.data(name='X', shape=[1], dtype='float32')
+	hidden = fluid.layers.fc(input=data, size=10)
 	layers.assign(hidden, out)
-	loss = layers.mean(out)
+	loss = fluid.layers.mean(out)
 	adam = fluid.optimizer.Adam()
 	adam.minimize(loss)
 

doc/fluid/api_cn/initializer_cn.rst

@@ -125,7 +125,7 @@ init_on_cpu
 .. code-block:: python
         
         with init_on_cpu():
-                step = layers.create_global_var()
+                step = fluid.layers.create_global_var()