add example for reshape op (#22129)

* add example for reshape op

python/paddle/fluid/layers/nn.py

@@ -5556,6 +5556,12 @@ def reshape(x, shape, actual_shape=None, act=None, inplace=False, name=None):
             dim = fluid.layers.fill_constant([1], "int32", 5)
             reshaped_2 = fluid.layers.reshape(data_2, shape=[dim, 10])
             # the shape of reshaped_2 is [5,10].
+
+            # example 3:
+            data_3 = fluid.data(
+              name="data_3", shape=[2,4,6], dtype='float32')
+            reshaped_3 = fluid.layers.reshape(x=data_3, shape=[6,8])
+            # the shape of reshaped_3 is [6,8].
     """
     if in_dygraph_mode():
         #TODO(zhiqiu): open inplace if we can.