Support callback

python/paddle/v2/fluid/backward.py

@@ -188,7 +188,10 @@ def _append_backward_ops_(target,
         grad_to_var(dict)(output argument):
             key(str): grad variable name
             val(str): corresponding forward variable name
+        callback(callable object): a callable object used to decorate new generated grad ops
     """
+    if callback is not None and not hasattr(callback, '__call__'):
+        raise ValueError("'callback' must be a callable object.")
     # grad_op_descs holds created grad_op, and will be appended to target_block
     grad_op_descs = []
     program = block.program
@@ -205,6 +208,8 @@ def _append_backward_ops_(target,
         # Getting op's corresponding grad_op
         grad_op_desc, op_grad_to_var = core.get_grad_op_desc(
             op.desc, no_grad_dict[block.idx], grad_sub_block_list)
+        if callback is not None:
+            grad_op_desc = callback(grad_op_desc)
         grad_op_descs.extend(grad_op_desc)
         grad_to_var.update(op_grad_to_var)