CuPy support

[flying-sheep]

Fixes #3

Intended properties:

• like with Dask, functions taking cupy arrays or matrices should mostly return cupy arrays (except things like as_dense(to_memory=True) ofc
• unlike with Dask, scalars should get returned as scalars, not arrays (no need to keep a single number in GPU memory)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.45%. Comparing base (7319f89) to head (39a9da2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
+ Coverage   96.87%   98.45%   +1.58%     
==========================================
  Files          15       15              
  Lines         256      259       +3     
==========================================
+ Hits          248      255       +7     
+ Misses          8        4       -4     

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[codspeed-hq]

CodSpeed Performance Report

Merging #51 will improve performances by ×2.9

_{Comparing pa/cupy (39a9da2) with main (7319f89)}

Summary

⚡ 1 improvements
✅ 87 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_stats_benchmark[1-scipy.sparse.csr_array-float32-is_constant]	355.2 µs	123.9 µs	×2.9

[ilan-gold]

This PR seems to mostly just add types and a sum function? Is the reason that this all just "works" because cupy handles numpy well?

[ilan-gold]

In general I'm getting a bit concerned about all the casting going on but since it's in pre-commit, it should be manageable from a us-reviewing-other-PRs perspective i.e., we can always just tell people to look at the output. I was a little worried that there was no CI around this

[flying-sheep]

Casting? You mean mypy? I think mypy and Python’s type system are simply not that great.

With TypeScript, I could rely on the type system saying the truth, but in Python it’s more often an error in the typing or a bug in mypy than my fault.

Makes the whole ordeal of rather limited use tbh if one can’t rely on the type system.

Maybe switching to PyRight could help.