* fix: remove all module wrapper when saving checkpoint
* refactor: move position of if
* docs: add docstring
* refactor: add _save_to_state_dict from official torch
* docs: modify docstring of _save_to_state_dict
* docs: modify docstring
* feat: add unittest
* feat: add DataParallel to unittest
* fix: a bug when model has batchnorm
* docs: update docstring
