use len instead of size for python list

Author: jacquesqiao

python/paddle/fluid/transpiler/distribute_transpiler.py

@@ -767,7 +767,7 @@ 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:
+        if len(optimize_blocks) == 0:
             pre_block_idx = pserver_program.num_blocks - 1
             empty_block = pserver_program._create_block(pre_block_idx)
             optimize_blocks.append(empty_block)