[mt] Cleanup partitioners.

[trivialfis]

• Share the initialization code between sc tree and mt tree updaters.
• Share the temporary buffer between partitioners.
• Implement external memory support for reduced gradient.

ref #9043

[Copilot]

Pull Request Overview

This PR refactors the GPU histogram tree builder code by introducing a RowPartitionerBatches wrapper struct to consolidate partitioner management logic and improves code clarity by renaming node_sum to child_sum in split candidate structures.

Key changes:

• Introduces RowPartitionerBatches struct to encapsulate partitioner vector operations with a cleaner API
• Renames node_sum to child_sum throughout multi-target split code for better semantic clarity
• Removes dead code (unused sum_zeros calculation) and redundant checks

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/tree/gpu_hist/row_partitioner.cuh	Adds new RowPartitionerBatches struct with Init method and accessor functions to wrap partitioner vector management
src/tree/updater_gpu_hist.cuh	Replaces std::vector<std::unique_ptr<RowPartitioner>> with RowPartitionerBatches type and updates method calls to use new API (At, Size, Empty)
src/tree/updater_gpu_hist.cu	Similar updates as .cuh file, replacing vector operations with RowPartitionerBatches methods
src/tree/updater_gpu_common.cuh	Renames node_sum member to child_sum in MultiSplitCandidate struct
src/tree/gpu_hist/multi_evaluate_splits.cu	Updates references from node_sum to child_sum and removes unused sum_zeros calculation
src/tree/gpu_hist/expand_entry.cu	Updates output stream operator to use child_sum instead of node_sum
tests/cpp/tree/gpu_hist/test_multi_evaluate_splits.cu	Updates test assertions to check child_sum instead of node_sum
python-package/xgboost/testing/multi_target.py	Adds min_cache_page_bytes parameter to test iterator configuration for better external memory testing

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[Copilot]

Pull Request Overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

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[trivialfis]

cc @rongou