update shape to make it meaningfull, test=develop (#1964)

doc/fluid/api_cn/layers_cn/pad2d_cn.rst

@@ -55,5 +55,5 @@ pad2d
 .. code-block:: python
 
     import paddle.fluid as fluid
-    data = fluid.data(name='data', shape=[1, 1, 2, 3], dtype='float32')
+    data = fluid.data(name='data', shape=[None, 3, 32, 32], dtype='float32')
     result = fluid.layers.pad2d(input=data, paddings=[0, 1, 2, 3], mode='reflect')

doc/fluid/api_cn/layers_cn/pad_cn.rst

@@ -42,7 +42,7 @@ pad
 
     # x 为一个秩为2的张量
     import paddle.fluid as fluid
-    x = fluid.data(name='data', shape=[2, 2], dtype='float32')
+    x = fluid.data(name='data', shape=[300, 300], dtype='float32')
     out = fluid.layers.pad(x=x, paddings=[0, 1, 1, 2], pad_value=0.)