improvements to chunk

[CarloLucibello]

• make chunk accept a collection of sizes
• use view instead of selectdim in order to improve returned type for cuda arrays

Fix #114, fix #103

[codecov-commenter]

Codecov Report

Merging #133 (f8c8f51) into main (08ad0b7) will decrease coverage by 0.43%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
- Coverage   88.61%   88.18%   -0.44%     
==========================================
  Files          13       15       +2     
  Lines         571      584      +13     
==========================================
+ Hits          506      515       +9     
- Misses         65       69       +4     

Impacted Files	Coverage Δ
src/utils.jl	89.70% <89.47%> (-0.70%)	⬇️
src/Datasets/Datasets.jl	0.00% <0.00%> (ø)
src/MLUtils.jl	0.00% <0.00%> (ø)

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