update

python/paddle/v2/fluid/backward.py

 from paddle.v2.fluid import framework as framework
+from . import core
 
 __all__ = ['append_backward_ops']
 
 
+def backward_impl(block, target_block, no_grad_set, grad_to_var, callback):
+    grad_op_descs = []
+    program = block.program
+    for each_op in block.ops:
+        grad_sub_block_list = []
+        if each_op.has_attr("sub_block"):
+            sub_block_idx = each_op.block_attr("sub_block")
+            sub_block = program.block(sub_block_idx)
+            grad_sub_block = program.create_block(parent_idx=sub_block_idx)
+            backward_impl(sub_block, grad_sub_block, no_grad_set, grad_to_var,
+                          callback)
+            grad_sub_block_list.append(grad_sub_block)
+        grad_op_desc = core.get_grad_op_desc(each_op.desc,
+                                             no_grad_set[block.idx],
+                                             grad_to_var, grad_sub_block_list)
+        grad_op_descs.append(grad_op_desc)
+
+
 def append_backward_ops(loss, parameter_list=None, no_grad_set=None):
     """
     Create and add gradient Operators in BlockDesc to compute

python/paddle/v2/fluid/framework.py

@@ -806,9 +806,11 @@ class Program(object):
         self.sync_with_cpp()
         return param_to_grad_info
 
-    def create_block(self):
+    def create_block(self, parent_idx=None):
         new_block_idx = len(self.blocks)
-        self.desc.append_block(self.current_block().desc)
+        parent = self.current_block() if parent_idx is None else self.block(
+            parent_idx)
+        self.desc.append_block(parent.desc)
         self.current_block_idx = new_block_idx
         self.blocks.append(Block(self, self.current_block_idx))
         return self.current_block()