* add new load dygraph func

* update load_pretrain_params

* update load_dygrah_params

* Update save_load.py

* Update train.py

* Update save_load.py

* return {} when path is None

* return {} when path is None

Fixed variable name

Co-authored-by: Nlittletomatodonkey <2120160898@bit.edu.cn>

* fix loader cause it does not need to be rebuild for each epoch

* fix loader for dynamic sample

* rm thin import