* feat: add custom_group to DefaultOptimConstrutor
* refactor: move custom_groups validate to _validate_cfg
* docs: add doc to explain custom_groups
* feat: add unittest for non_exist_key
* refactor: one param per group
* fix: small fix
* fix: name
* docs: docstring
* refactor: change to mult for only lr and wd custom
* docs: docstring
* docs: more explaination
* feat: sort custom key
* docs: add docstring
* refactor: use reverse arg of sorted
* docs: fix comment
* docs: fix comment
* refactor: small modi
* refactor: small modi
* refactor: small modi