Move buffers into decision_forest, to avoid triggering copy constructor

[chyunsu3]

Currently, buffer arguments to decision_forest are passed to member variables without std::move, triggering the copy constructor. This behavior is not intended, since the arguments are all rvalues (&&).

The extra buffer copies are problematic because the copy constructor uses the default stream. If the initial host-to-device copy occurs on a non-default stream, then the copy construction is unordered with respect to the host-to-device copy (i.e. may occur before or after). So the forest model may end up with garbage value.

Fix. Ensure that the buffer arguments are moved into the decision_forest. This way, we do not make extra copies after the initial host-to-device copy.

I found this bug while working on #193, since it was using a non-default stream.

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Performance
  • Improved model and forest construction efficiency by reducing unnecessary data copying.
  • Preserved existing validation and device initialization behavior while transferring ownership more efficiently.

Walkthrough

The constructors now move forest buffers and variants into owned members. Validation and device initialization use the transferred buffers.

Changes

Forest construction

Layer / File(s)	Summary
Decision forest buffer ownership cpp/include/nvforest/decision_forest.hpp	The constructor move-initializes forest buffers. Validation and device initialization use the stored members.
Forest model variant ownership cpp/include/nvforest/forest_model.hpp	The constructor move-constructs decision_forest_ from the supplied variant.

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

Mergeability Score: ⚪ Minimal · up to e70da

The change moves rvalue buffers into the model to avoid unintended copies and preserve stream ordering. The PR is merge-ready after normal checks; updating the public ownership documentation remains a minor follow-up.

Possibly related PRs

• rapidsai/nvforest#132: This PR modifies the same constructors, with different ownership semantics.
• rapidsai/nvforest#193: This PR also updates these constructors to move inputs.

Suggested labels: improvement

Suggested reviewers: csadorf, ramitchell

🚥 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.
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 identifies the main change: moving buffers into decision_forest to avoid unintended copies.
Description check	✅ Passed	The description accurately explains the copy issue, stream-ordering risk, and move-based fix.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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