move checkpoint api to trainer.py
move checkpoint api to trainer.py

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