LightGBM

[dask] Add type hints in Dask package (#3866)

* add type hints in dask module

* starting on asserts

* remove unused code

* add hints for dtypes

* replace accidentally-removed docstrings

* revert unrelated change

* Update python-package/lightgbm/dask.py

* empty commit

* fix hints on group

* capitalize array

* hide hints in signatures

* empty commit

* sphinx version

* Apply suggestions from code review

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

* fix hint for MatrixLike

* Update python-package/lightgbm/dask.py

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

* Apply suggestions from code review

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

* update docstring

* empty commit

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