[Feature] Add empty cache hook (#58)

* [Feature]: Add Part3 of Hooks

* [Feature]: Add Hook

* [Fix]: Add docstring and type hint for base hook

* [Fix]: Add test case to not the last iter, inner_iter, epoch

* [Fix]: Add missing type hint

* [Feature]: Add Args and Returns in docstring

* [Fix]: Add missing colon

* [Fix]: Add optional to docstring

* [Fix]: Fix docstring problem

* [Fix]: Fix lint

* fix lint

* update typing and docs

* fix lint

* Update mmengine/hooks/empty_cache_hook.py

Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* Update mmengine/hooks/empty_cache_hook.py

Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* Update mmengine/hooks/empty_cache_hook.py

Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* Update tests/test_hook/test_empty_cache_hook.py

Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* fix lint

* fix comments

* remove test condition

Co-authored-by: seuyou <3463423099@qq.com>
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>