perf: run umap in parallel when rng=None is passed

[flying-sheep]

• Closes Umap parallel #3211
• Tests included or not required because: preformance-only change, see benchmark

• Release notes not necessary because:

milestone 1.13.0, since 1.12.x doesn’t have rng.

scanpy 2.0 will have rng=None by default and therefore the speedup.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.50%. Comparing base (9bc2c1e) to head (f51185e).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4036      +/-   ##
==========================================
- Coverage   78.51%   78.50%   -0.01%     
==========================================
  Files         117      117              
  Lines       12753    12750       -3     
==========================================
- Hits        10013    10010       -3     
  Misses       2740     2740              

Flag	Coverage Δ
hatch-test.low-vers	77.80% <100.00%> (-0.01%)	⬇️
hatch-test.pre	77.46% <100.00%> (+12.62%)	⬆️

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

Files with missing lines	Coverage Δ
src/scanpy/tools/_umap.py	70.96% <100.00%> (-1.34%)	⬇️

[scverse-benchmark]

Benchmark changes

Change	Before [9bc2c1e]	After [f51185e]	Ratio	Benchmark (Parameter)
-	1.04±0s	340±2ms	0.33	tools.ToolsSuite.time_umap

Comparison: https://github.com/scverse/scanpy/compare/9bc2c1ed1bd1cf6f6c06cec71cce99916d048163..f51185ef4c38bd72b9f6e6647c4d74515507dfb8
Last changed: Tue, 7 Apr 2026 13:55:40 +0000

More details: https://github.com/scverse/scanpy/pull/4036/checks?check_run_id=70245416359