* [Enhance] support eta_min_ratio in CosineAnnealingParamScheduler

* [doc] fix docstring

* [Enhance] add ut for eta_min_ratio

* [doc] update docstring

* avoid bc-breaking of eta_min

* [doc] add docstring in CosineAnnealingParamScheduler and CosineAnnealingMomentum

* Apply suggestions from code review

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