fix the default value prefetch_var_name_to_block_id

paddle/fluid/operators/listen_and_serv_op.cc

@@ -340,7 +340,8 @@ class ListenAndServOpMaker : public framework::OpProtoAndCheckerMaker {
     AddAttr<framework::BlockDesc *>(kOptimizeBlock,
                                     "BlockID to run on server side.");
     AddAttr<std::vector<std::string>>(kPrefetchVarNameToBlockId,
-                                      "prefetch block to run on server side.");
+                                      "prefetch blocks to run on server side.")
+        .SetDefault({});
     AddAttr<int>("Fanin", "How many clients send to this server.")
         .SetDefault(1);
   }

python/paddle/fluid/transpiler/distribute_transpiler.py

@@ -530,19 +530,23 @@ class DistributeTranspiler:
         else:
             assert len(prefetch_var_name_to_block_id) == 0
 
+        attrs = {
+            "OptimizeBlock": pserver_program.block(1),
+            "endpoint": endpoint,
+            "Fanin": self.trainer_num,
+            "sync_mode": self.sync_mode,
+            "grad_to_block_id": grad_to_block_id
+        }
+        if len(prefetch_var_name_to_block_id) > 0:
+            attrs['prefetch_var_name_to_block_id'] \
+                = prefetch_var_name_to_block_id
+
         # step5 append the listen_and_serv op
         pserver_program.global_block().append_op(
             type="listen_and_serv",
             inputs={'X': recv_inputs},
             outputs={},
-            attrs={
-                "OptimizeBlock": pserver_program.block(1),
-                "endpoint": endpoint,
-                "Fanin": self.trainer_num,
-                "prefetch_var_name_to_block_id": prefetch_var_name_to_block_id,
-                "sync_mode": self.sync_mode,
-                "grad_to_block_id": grad_to_block_id
-            })
+            attrs=attrs)
 
         pserver_program.sync_with_cpp()
         return pserver_program