 Fix model average on multi-GPUs. (#11814)
 Fix model average on multi-GPUs. (#11814)

* Fix average_accumulate_op for parallel executor.

* Fix model average on multi-GPUs.

tensor-tang/Paddle,PaddlePaddle/Paddle,QiJune/Paddle,baidu/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,jacquesqiao/Paddle,PaddlePaddle/Paddle,reyoung/Paddle,jacquesqiao/Paddle,baidu/Paddle,QiJune/Paddle,reyoung/Paddle,luotao1/Paddle,reyoung/Paddle,luotao1/Paddle,reyoung/Paddle,tensor-tang/Paddle,tensor-tang/Paddle,jacquesqiao/Paddle,PaddlePaddle/Paddle,chengduoZH/Paddle,reyoung/Paddle,chengduoZH/Paddle,QiJune/Paddle,jacquesqiao/Paddle,jacquesqiao/Paddle,PaddlePaddle/Paddle,baidu/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,QiJune/Paddle,luotao1/Paddle,tensor-tang/Paddle,QiJune/Paddle,luotao1/Paddle,chengduoZH/Paddle,jacquesqiao/Paddle,chengduoZH/Paddle,QiJune/Paddle,reyoung/Paddle,luotao1/Paddle,luotao1/Paddle,chengduoZH/Paddle,baidu/Paddle,baidu/Paddle