set en empty optimize block if pserver has no optimize block

Author: jacquesqiao

python/paddle/fluid/transpiler/distribute_transpiler.py

@@ -767,6 +767,13 @@ def __clone_lr_op_sub_block__(op, program, lr_block):
             prefetch_var_name_to_block_id.extend(
                 lookup_table_var_name_to_block_id)
 
+        if optimize_blocks.size() == 0:
+            pre_block_idx = pserver_program.num_blocks - 1
+            empty_block = pserver_program._create_block(pre_block_idx)
+            optimize_blocks.append(empty_block)
+
+        # In some case, some parameter server will have no parameter to optimize
+        # So we give an empty optimize block to parameter server.
         attrs = {
             "optimize_blocks": optimize_blocks,
             "endpoint": endpoint,