Dev/update ptb (#4025)

* update ptb model to use auto generated op functions, test=develop

* refine model code, test=develop

* refine model code, test=develop

dygraph/ptb_lm/ptb_dy.py

@@ -223,7 +223,6 @@ class PtbModel(fluid.Layer):
         loss = fluid.layers.reshape(loss, shape=[-1, self.num_steps])
         loss = fluid.layers.reduce_mean(loss, dim=[0])
         loss = fluid.layers.reduce_sum(loss)
-        loss.permissions = True
 
         return loss, last_hidden, last_cell