Remove unnecessary var_and_op of DynamicRnn (#14134)

* remove unnecessary var_and_op
test=develop

* fix _init_zero_idx_
test=develop

python/paddle/fluid/layers/control_flow.py

@@ -1586,8 +1586,7 @@ class DynamicRNN(object):
         self.lod_rank_table = None
         self.max_seq_len = None
         self.step_idx = None
-        self.zero_idx = fill_constant(
-            shape=[1], value=0, dtype='int64', force_cpu=True)
+        self.zero_idx = None
         self.mem_dict = dict()
         self.output_array = []
         self.outputs = []
@@ -1792,6 +1791,7 @@ class DynamicRNN(object):
 
         """
         self._assert_in_rnn_block_('memory')
+        self._init_zero_idx_()
         if init is not None:
             if not isinstance(init, Variable):
                 raise TypeError(
@@ -1905,6 +1905,22 @@ class DynamicRNN(object):
             array_write(x=each, i=self.step_idx, array=outside_array)
             self.output_array.append(outside_array)
 
+    def _init_zero_idx_(self):
+        if self.zero_idx is None:
+            parent_block = self._parent_block_()
+            self.zero_idx = parent_block.create_var(
+                name=unique_name.generate('zero_idx'), dtype='int64')
+            parent_block.append_op(
+                type='fill_constant',
+                inputs={},
+                outputs={'Out': [self.zero_idx]},
+                attrs={
+                    'shape': [1],
+                    'dtype': self.zero_idx.dtype,
+                    'value': float(0),
+                    'force_cpu': True
+                })
+
     def _parent_block_(self):
         prog = self.helper.main_program
         parent_idx = prog.current_block().parent_idx