Fix segmented gather for sliced gather map input

[davidwendt]

Description

Fixes the output for cudf::lists::segmented_gather when the input gather_map_list is sliced. The output offsets are adjusted so that offset[0]=0 only when the gather_map.offset > 0.
The gtests are updated to ensure this case is checked.

Closes #23826

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Corrected sliced gather operations so output offsets consistently begin at zero.
  • Improved handling of sliced gather results, including cases with out-of-bounds values configured to become null.
  • Updated result validation to require exact column matches, improving accuracy and consistency.

Walkthrough

segmented_gather now normalizes offsets copied from sliced gather maps by subtracting the sliced base offset. Tests use exact column comparisons for valid and NULLIFY out-of-bounds sliced gathers.

Changes

Segmented gather offset normalization

Layer / File(s)	Summary
Normalize sliced gather offsets cpp/src/lists/copying/segmented_gather.cu	Adds RMM and Thrust support, then subtracts the original base offset from copied offsets when the gather map is sliced.
Validate canonical sliced results cpp/tests/copying/segmented_gather_list_tests.cpp	Uses exact column comparisons for valid and NULLIFY out-of-bounds sliced gathers. Updates the copyright notice.

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

Merge Risk: 🔵 Low · up to 81c2a

The fix adjusts segmented-gather offsets for sliced inputs, but temporary allocations may use a different memory resource than the caller requested. The change is otherwise bounded and mergeable with explicit owner awareness or follow-up to pass the requested resource.

Suggested reviewers: pointkernel, vuule, vyasr

🚥 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%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files.	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 and concisely describes the fix for sliced gather-map input, which is the primary change.
Description check	✅ Passed	The description accurately explains the offset correction, test updates, and linked issue.
Linked Issues check	✅ Passed	The changes subtract the first copied offset to produce canonical output offsets for sliced gather maps. The tests use exact equality to validate the corrected offsets. These changes satisfy issue #23…
Out of Scope Changes check	✅ Passed	The code and test changes remain within scope. The copyright update is a routine metadata change, and the test updates directly validate the fix.

Full details: Linked Issues check

Explanation

The changes subtract the first copied offset to produce canonical output offsets for sliced gather maps. The tests use exact equality to validate the corrected offsets. These changes satisfy issue #23826.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@cpp/src/lists/copying/segmented_gather.cu`:
- Line 106: Update the thrust::transform execution policy in the segmented
gather implementation to pass the caller-provided mr memory resource to
rmm::exec_policy_nosync, ensuring temporary allocations use that resource
instead of the current device resource.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 07d7160 and 81c2afb.

📒 Files selected for processing (2)

• cpp/src/lists/copying/segmented_gather.cu
• cpp/tests/copying/segmented_gather_list_tests.cpp

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