* update evaluator prefix

* update docstring and comments

* update doc

* Support default_scope when building optimizer and evaluator.

* add docstring

* fix

* fix

* update type hint and ut for evaluator

* update doc

* fix

* support metric prefix in BaseEvaluator

* update docs

* use class attribute default_prefix

* [Feature]: Add evaluator base class.

* solve comments

* update

* fix