Fallback unsupported cuml.accel PCA and KernelRidge cases to CPU

[csadorf]

This updates cuml.accel to fall back to CPU for two sklearn-compatible cases that are unsupported on GPU: PCA(n_components=0) and KernelRidge with non-string sklearn kernels. The PCA(n_components=0) behavior could be emulated in cuML in a follow-up, but the practical benefit is likely small since it is mainly an edge-case parity behavior. This also documents the new fallback limitations.

Contributes to #8180

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📥 Commits

Reviewing files that changed from the base of the PR and between 83a391f and c3b1eca.

📒 Files selected for processing (1)

• python/cuml/cuml_accel_tests/upstream/scikit-learn/xfail-list.yaml

💤 Files with no reviewable changes (1)

• python/cuml/cuml_accel_tests/upstream/scikit-learn/xfail-list.yaml

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Updated GPU handling for PCA to reject n_components=0, while preserving CPU fallback behavior.
  • Improved KernelRidge GPU interop to require a string kernel; non-string (e.g., callable) kernels now trigger CPU fallback instead of attempting GPU execution.
• Tests
  • Added integration coverage for PCA(n_components=0) to confirm zero-dimensional outputs and learned attribute shapes.
  • Added integration coverage for KernelRidge with a scikit-learn kernel to verify finite predictions and expected fitted attributes.
• Documentation
  • Updated cuml.accel limitations documentation to reflect the new CPU-fallback conditions for PCA and KernelRidge.

Walkthrough

The PR adds CPU-fallback checks for PCA when n_components=0 and KernelRidge when kernel is not a string, and updates the limitations docs and integration tests for both cases.

Changes

CPU fallback checks for PCA and KernelRidge

Layer / File(s)	Summary
PCA GPU fallback guard python/cuml/cuml/decomposition/pca.pyx, docs/source/cuml-accel/limitations.rst, python/cuml/cuml_accel_tests/integration/test_pca.py, python/cuml/cuml_accel_tests/upstream/scikit-learn/xfail-list.yaml	PCA._params_from_cpu raises UnsupportedOnGPU for n_components=0, and the limitations text, integration test, and xfail list updates cover that case.
KernelRidge CPU fallback guard python/cuml/cuml/kernel_ridge/kernel_ridge.py, docs/source/cuml-accel/limitations.rst, python/cuml/cuml_accel_tests/integration/test_kernel_ridge.py	KernelRidge._params_from_cpu raises UnsupportedOnGPU when kernel is not a string, and the limitations text and integration test cover that path.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested labels

cuml-accel, improvement

Suggested reviewers

• jcrist
• gforsyth

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: adding CPU fallbacks for unsupported PCA and KernelRidge cases in cuml.accel.
Description check	✅ Passed	The description accurately describes the new CPU fallbacks and documentation updates in this changeset.
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.

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[csadorf]

/merge