* [Enhancement] Add PolyParamScheduler, PolyMomentum and PolyLR

* min_lr -> eta_min, refined docstr

* Support default_scope when building optimizer and evaluator.

* add docstring

* fix

* fix

* [Feature]: Add optimzier and constructor.

* refactor unit tests

* add optimizer doc

* add parrots wrapper

* add parrots wrapper

* solve comments

* resolve comments

* [Feature]: Add parameter schedulers.

* update

* update

* update

* update

* add docstring to lr and momentum

* resolve comments