perf: welford's algorithm for mean-var aggregation

[ilan-gold]

From discussions with @zboldyga.

This should then in theory be reused with #4143 instead of its custom moments calculation

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[ilan-gold]

We should make these nogil or provide an option for fau to provide nogil njit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.60%. Comparing base (0ac3337) to head (48230af).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4147      +/-   ##
==========================================
- Coverage   79.61%   79.60%   -0.02%     
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  Files         120      120              
  Lines       12786    12780       -6     
==========================================
- Hits        10180    10173       -7     
- Misses       2606     2607       +1     

Flag	Coverage Δ
hatch-test.low-vers	78.84% <100.00%> (-0.01%)	⬇️
hatch-test.pre	79.43% <100.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/get/_aggregated.py	92.66% <100.00%> (-0.65%)	⬇️
src/scanpy/get/_kernels.py	100.00% <ø> (ø)

[zboldyga]

@ilan-gold I reviewed since this plays into our other work

I don't see any issues with the welfords implementation, lgtm!

and overall I'm new with njit, OMP, TBB. But I see your point about nogil, and was able to create some scenarios that trigger that path. So agreed on the suggestion... I also found that it does also slightly improve the chan njit work when fau falls back on the serial build, and I guess it would generally fix any of these types of things across scanpy, so maybe that does belong in fau?

There's probably some threading stuff I'm not fully grasping with scanpy, fau, those libraries yet though; I will be more confident in that in a few weeks. I figure I will revisit threading and numba as a whole as part of building a good understanding of these libraries, and if I find any thread issues throughout scanpy/fau I will raise them at that point.

[ilan-gold]

so maybe that does belong in fau?

Yes the issue with nogil is that it means your code is no longer threadsafe with respect to its inputs (in a certain mental model of things, I guess). So introducing this change needs some though - I don't think anything in FAU actually alters its inputs though so we should be good.

There's probably some threading stuff I'm not fully grasping with scanpy, fau, those libraries yet though; I will be more confident in that in a few weeks. I figure I will revisit threading and numba as a whole as part of building a good understanding of these libraries, and if I find any thread issues throughout scanpy/fau I will raise them at that point.

That would be amazing because it is something we have (clearly) struggled with