Merge pull request #32 from Rishat-skoltech/rapids_new

merge conflict

minor code refactoring

added fill_na with cp.nan to avoid bugs with null columns

fixed bug with NA values in two-layer pipelines

commit linear models

minor fixes

Update categorical_gpu.py

Update base.py

Update tabular_gpu_presets.py

changed reader to cudf

removed prints

fixed linear model dsitributed

added gpu import check to main file of gpu preset

fixes with reader, bug fix, import fixes