LightGBM

[python] add return_cvbooster flag to cv func and publish _CVBooster (#283,#2105,#1445) (#3204)

* [python] add return_cvbooster flag to cv function and rename _CVBooster to make public (#283,#2105)

* [python] Reduce expected metric of unit testing

* [docs] add the CVBooster to the documentation

* [python] reflect the review comments

- Add some clarifications to the documentation
- Rename CVBooster.append to make private
- Decrease iteration rounds of testing to save CI time
- Use CVBooster as root member of lgb

* [python] add more checks in testing for cv

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* [python] add docstring for instance attributes of CVBooster

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* [python] fix docstring

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>