Simplify dask serializers

[jcrist]

• Removes serializer override for MultinomialNB, this is unnecessary
• Simplifies the registration of serializers for other types, and improves the comments to better state why overrides are needed.

This is a small followup to #8385. I noticed in that PR that some of the code here could be simplified/was no longer necessary.

This should have no runtime effects (the same effective code should run before and after), it's just a small code cleanup.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 0277bdf and 73cdfbc.

📒 Files selected for processing (1)

• python/cuml/cuml/comm/serialize.py

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📝 Walkthrough

Summary by CodeRabbit

• Refactor
  • Streamlined internal serialization registration across supported distributed and GPU workflows.
  • Updated random forest models to use pickle-based serialization.
  • Removed obsolete serializer registrations without changing the public API.

Walkthrough

The serialization module replaces individual registrations with loop-based cuml.Base registration for Dask and CUDA serializers. Random forest classifier and regressor classes use explicit pickle overrides. MultinomialNB generic registrations are removed.

Changes

Serialization registration

Layer / File(s)	Summary
Consolidate serializer registrations python/cuml/cuml/comm/serialize.py	A loop registers Dask and CUDA serializers for cuml.Base. Random forest classifier and regressor classes explicitly use pickle serialization. Previous individual functions and MultinomialNB registrations are removed.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: divyegala

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: simplifying Dask serializer registration.
Description check	✅ Passed	The description accurately explains the serializer cleanup, removed override, clarified comments, and intended lack of runtime effects.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[viclafargue]

Thanks! LGTM

[jcrist]

/merge