[REVIEW] Fix intermittent dask random forest failure

[Nanthini10]

Adding n_classes=1 when n_samples=1 to ensure that class is always labelled 0.

Closes #3202

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[JohnZed]

Weird case! But fix looks good

[codecov-io]

Codecov Report

Merging #3239 (5739fe7) into branch-0.17 (a0f4549) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.17    #3239      +/-   ##
===============================================
- Coverage        71.48%   71.35%   -0.14%     
===============================================
  Files              206      206              
  Lines            16645    16879     +234     
===============================================
+ Hits             11899    12044     +145     
- Misses            4746     4835      +89     

Impacted Files	Coverage Δ
python/cuml/dask/neighbors/kneighbors_regressor.py	25.92% <0.00%> (-3.93%)	⬇️
...ython/cuml/dask/neighbors/kneighbors_classifier.py	18.60% <0.00%> (-2.70%)	⬇️
python/cuml/experimental/explainer/kernel_shap.pyx	97.74% <0.00%> (+0.48%)	⬆️
python/cuml/fil/fil.pyx	94.59% <0.00%> (+0.84%)	⬆️

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[JohnZed]

rerun tests

[JohnZed]

rerun tests