[REVIEW] Enable pipeline usage for OneHotEncoder and LabelEncoder

[viclafargue]

Answers #3053

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[viclafargue]

rerun tests

[codecov-io]

Codecov Report

Merging #3192 (7df31b9) into branch-0.17 (f1abcb8) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           branch-0.17    #3192      +/-   ##
===============================================
+ Coverage        70.99%   71.00%   +0.01%     
===============================================
  Files              198      198              
  Lines            15729    15729              
===============================================
+ Hits             11167    11169       +2     
+ Misses            4562     4560       -2     

Impacted Files	Coverage Δ
python/cuml/preprocessing/LabelEncoder.py	92.98% <100.00%> (ø)
python/cuml/preprocessing/encoders.py	95.10% <100.00%> (ø)
...hirdparty/sklearn/preprocessing/_discretization.py	85.60% <0.00%> (+1.51%)	⬆️

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[wphicks]

LGTM