add threads in thread pool for CPU (#12475)

Author: chengduo

python/paddle/fluid/parallel_executor.py

@@ -121,7 +121,7 @@ def __init__(self,
             else:
                 cpu_num = int(
                     os.environ.get('CPU_NUM', multiprocessing.cpu_count()))
-                exec_strategy.num_threads = cpu_num
+                exec_strategy.num_threads = cpu_num * 2
 
         if build_strategy is None:
             build_strategy = BuildStrategy()