LightGBM

[python-package] [dask] Add DaskLGBMRanker (#3708)

* ranker support wip

* fix ranker tests

* fix _make_ranking rnd gen bug, add sleep to help w stoch binding port failed exceptions

* add wait_for_workers to prevent Binding port exception

* another attempt to stabilize test_dask.py

* requested changes: docstrings, dask_ml, tuples for list_of_parts

* fix lint bug, add group param to test_ranker_local_predict

* decorator to skip tests with errors on fixture teardown

* remove gpu ranker tests, reduce make_ranking data complexity

* another attempt to
silence client, decorator does not silence fixture errors

* address requested changes on 1/20/20

* skip test_dask for all GPU tasks

* address changes requested on 1/21/21

* issubclass instead of __qualname__

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

* parity in group docstr with sklearn

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

* _make_ranking docstr cleanup

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

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