Fix incorrect async initialisations

[betatim]

These three locations involve a RMM API that is async, this means the values need to be kept alive until the stream is sync'ed.

The fix was created by AI. From looking at when the failures started appearing in the nightly CI it looked like github.com/rapidsai/rmm/pull/2511 was a candidate for the failures we see. Reading rapidsai/rmm#2521 makes me think these APIs were always used incorrectly by cuml, but the implementation on the inside was not taking full advantage of all the async'ness that it could. Hence we didn't see this until now.

This is also what the AI came up with and it had a plausible explanation of why this explains the failures. For example in this snippet the value in val is changed before set_value_async has used it.

value_t val = std::numeric_limits<value_t>::max();
min_d.set_value_async(val, stream);          // deferred read
val = std::numeric_limits<value_t>::lowest(); // val overwritten before the copy runs
max_d.set_value_async(val, stream);

The fix in cd.cuh makes sense as well. The fix in algo.cuh looks sensible, but I'd have to do a bit more thinking to be able to explain why/what it exactly does. I'm inclined to believe my friend AI on this though.

AI also had to do quite a lot of trickery (for a novice like me) to reproduce this issue locally on a non GB300. Which makes some amount of sense given we don't see this for jobs that don't use GB300. I can share the snippet it came up with in order to reproduce this locally. Not sure it is that useful.

Fixes part of #8510
Closes #8509 #8508

(I couldn't come up wit ha good title for this PR :( )

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved the reliability of GPU-based model fitting and embedding calculations.
  • Fixed asynchronous initialization issues that could cause inconsistent results during dimensionality reduction.
  • Corrected matrix operation handling to improve numerical accuracy and stability.
  • Ensured outlier-related calculations start from a consistent zero state.

Walkthrough

The changes update cuBLAS scalar handling and GEMV mode in coordinate descent, preserve separate host initialization values in t-SNE, and explicitly zero a UMAP device scalar.

Changes

Coordinate descent GEMV

Layer / File(s)	Summary
Host scalar GEMV update cpp/src/solver/cd.cuh	cdFit uses host cuBLAS scalar values, passes their addresses to GEMV, and selects non-transpose mode. The SPDX attribution includes NVIDIA affiliates.

t-SNE initialization

Layer / File(s)	Summary
Asynchronous min-max initialization cpp/src/tsne/fft_tsne.cuh	min_max uses separate constant host values for asynchronous minimum and maximum initialization.

UMAP outlier initialization

Layer / File(s)	Summary
Explicit outlier zeroing cpp/src/umap/simpl_set_embed/algo.cuh	has_outlier_d is constructed with the stream and explicitly zeroed. The SPDX attribution includes NVIDIA affiliates.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🔵 Low · up to 87bd5

The code change is limited to internal scalar storage and does not alter the public API; merge readiness is otherwise unaffected, but the required non-breaking label should be added before merge.

Suggested reviewers: chyunsu3, divyegala, dantegd

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
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.
Title check	✅ Passed	The title clearly summarizes the main change: correcting incorrect asynchronous initializations in the affected RMM API calls.
Description check	✅ Passed	The description directly explains the asynchronous initialization issue, the affected locations, the suspected RMM behavior, and the intended fixes.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (3 skipped: 3 unsupported.)

✨ Finishing Touches 💡 1

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@cpp/src/solver/cd.cuh`:
- Around line 216-218: Add the repository’s non-breaking label to PR `#8518`; no
source-code changes are required because the cdFit public signature remains
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📒 Files selected for processing (3)

• cpp/src/solver/cd.cuh
• cpp/src/tsne/fft_tsne.cuh
• cpp/src/umap/simpl_set_embed/algo.cuh

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[wence-]

nit: value(stream) synchronizes the stream, so you can drop the cudaStreamSynchronize (and naughty naughty, you should be checking the return code...)

[jcrist]

[jcrist]

/merge