* [Feature] Support overwrite default scope with "_scope_".

* add ut

* add ut

* add defaut scope

* Fix docstring

* override get_current_instance method in DefaultScope

clean meta nameing

* remove default mmengine argument of DefaltScope

remove default mmengine argument of DefaltScope

remove default mmengine argument of DefaltScope

* Fix unit test

Fix unit test

* Fix example in docstring

* add explaination of DefaultScope