Backport PR #4143 on branch 1.12.x (perf: chan's parallel mean-var algorithm for dask-backed arrays (sparse/dense))

[meeseeksmachine]

Backport PR #4143: perf: chan's parallel mean-var algorithm for dask-backed arrays (sparse/dense)

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.58%. Comparing base (7513959) to head (21441d7).
⚠️ Report is 1 commits behind head on 1.12.x.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/get/_aggregated.py	95.83%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           1.12.x    #4180      +/-   ##
==========================================
+ Coverage   78.54%   78.58%   +0.04%     
==========================================
  Files         121      121              
  Lines       12648    12685      +37     
==========================================
+ Hits         9934     9969      +35     
- Misses       2714     2716       +2     

Flag	Coverage Δ
hatch-test.low-vers	77.62% <39.58%> (-0.17%)	⬇️
hatch-test.pre	78.47% <95.83%> (+0.04%)	⬆️

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Files with missing lines	Coverage Δ
src/scanpy/get/_aggregated.py	92.94% <95.83%> (+0.28%)	⬆️