chore: add speed note to leiden warning

[ilan-gold]

• Closes leiden: warn about speed of igraph flavor being **much** faster than leidenalg flavor #3911
• Tests included or not required because:

• Release notes not necessary because: Just a warning

[codecov]

Codecov Report

❌ Patch coverage is 85.18519% with 4 lines in your changes missing coverage. Please review.
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Files with missing lines	Patch %	Lines
src/scanpy/tools/_leiden.py	85.18%	4 Missing ⚠️

Additional details and impacted files

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Flag	Coverage Δ
hatch-test.low-vers	77.59% <85.18%> (?)
hatch-test.pre	77.22% <85.18%> (?)

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Files with missing lines	Coverage Δ
src/scanpy/tools/_leiden.py	93.50% <85.18%> (ø)

[flying-sheep]

Hmm, two warnings when flavor=None … I think it’s OK, but maybe it would be more elegant to just add the text to the FutureWarning when raising that one, and only raise the UserWarning when specifying flavor="leidenalg" manually?

[flying-sheep]

why not do this more locally?

[ilan-gold]

Hmm, two warnings when flavor=None … I think it’s OK, but maybe it would be more elegant to just add the text to the FutureWarning when raising that one, and only raise the UserWarning when specifying flavor="leidenalg" manually?

But I think we want people definitely using igraph either way - if you don't specify anything and if you do, either way you should be using igraph.

[flying-sheep]

That’s why I suggested

maybe it would be more elegant to just add the [new] text to the FutureWarning when raising that one [and raising the new text as a standalone UserWarning otherwise]

that way, the user would get

• max 1 warning
• warning text that talks about future behehavior when relying on default behavior
• warning text that talks about performance when not using igraph