[REVIEW] Fix cuDF to cuPy conversion (missing value)

[viclafargue]

Answers #2966 .
This change should probably be implemented in cuDF. In the meantime, I modified the input_to_cupy_array function to produce a proper conversion of cuDF Dataframe and Series to cuPy array even when they contain missing values. The data is upcasted to float64 when necessary.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[dantegd]

Had a couple of questions

[codecov-io]

Codecov Report

Merging #3194 (c674d0a) into branch-0.17 (f1abcb8) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           branch-0.17    #3194      +/-   ##
===============================================
+ Coverage        70.99%   71.03%   +0.03%     
===============================================
  Files              198      198              
  Lines            15729    15737       +8     
===============================================
+ Hits             11167    11178      +11     
+ Misses            4562     4559       -3     

Impacted Files	Coverage Δ
python/cuml/thirdparty_adapters/adapters.py	88.55% <ø> (ø)
python/cuml/common/input_utils.py	93.30% <100.00%> (+0.76%)	⬆️
...hirdparty/sklearn/preprocessing/_discretization.py	85.60% <0.00%> (+1.51%)	⬆️

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[wphicks]

LGTM