提供多轮训练的demo

**System information** - DeepRec version (you are using): - Are you willing to contribute it (Yes/No):yes **Describe the feature and the current behavior/state.** 推荐一般需要回溯很多天的数据，进行eval/train 流式训练，可以提供多天流式训练的demo例子吗? 例如这种： https://github.com/PaddlePaddle/PaddleRec/blob/master/doc/online_trainer.md **Will this change the current api? How?** **Who will benefit with this feature?** **Any Other info.**