add sparse to gpu tests

[cTatu]

Description

Adding sparse handling for KNN and Matmul. Also simplifying distance computation on GPU for neighbors.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. If you haven't added any test and it is relevant provide instructions so we can reproduce.

• I have tested it manually in a local environment.
• I have tested it manually in a supercomputer.

[codecov]

Codecov Report

Merging #434 (a8b0173) into master (ffd8a9b) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
- Coverage   94.73%   94.71%   -0.02%     
==========================================
  Files          56       56              
  Lines        8621     8614       -7     
==========================================
- Hits         8167     8159       -8     
- Misses        454      455       +1     

Files	Coverage Δ
dislib/classification/knn/base.py	100.00% <ø> (ø)
dislib/math/base.py	82.49% <0.00%> (ø)
dislib/decomposition/qr/base.py	85.21% <0.00%> (ø)
dislib/cluster/kmeans/base.py	87.07% <0.00%> (+3.92%)	⬆️
dislib/neighbors/base.py	73.40% <0.00%> (+3.50%)	⬆️
dislib/data/array.py	93.92% <0.00%> (-1.50%)	⬇️

... and 1 file with indirect coverage changes

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