update parallel.py (#19371)

test=release/1.5

Author: chengduoZH

python/paddle/fluid/dygraph/parallel.py

@@ -188,16 +188,14 @@ def _split_tensors(self, coalesced_grads_and_grad_vars):
         from ..layers import nn
         for coalesced_grad, origin_grad_vars, grad_shapes in coalesced_grads_and_grad_vars:
             grad_var_len = [np.prod(g_shape) for g_shape in grad_shapes]
-            splited_vars = nn.split(
-                coalesced_grad, num_or_sections=grad_var_len, dim=0)
-            reshaped_grad_vars = []
-            for g_var, g_shape in zip(splited_vars, grad_shapes):
-                reshaped_grad_vars.append(
-                    nn.reshape(
-                        x=g_var, shape=g_shape, inplace=True))
-            for origin_g_var, reshaped_g_var in zip(origin_grad_vars,
-                                                    reshaped_grad_vars):
-                nn.assign(input=reshaped_g_var, output=origin_g_var)
+            self._helper.main_program.current_block().append_op(
+                type='split',
+                inputs={'X': coalesced_grad},
+                outputs={'Out': origin_grad_vars},
+                attrs={'sections': grad_var_len,
+                       'axis': 0})
+            for g_var, g_shape in zip(origin_grad_vars, grad_shapes):
+                nn.reshape(x=g_var, shape=g_shape, inplace=True)
 
     def apply_collective_grads(self):
         """