Use predictor for leaf id cache updates

[RAMitchell]

Summary

Move training prediction-cache accumulation from tree updaters into the predictor layer.

With updater row positions standardized, cache updates no longer need updater-specific implementations. This adds Predictor::PredictFromLeafIds, which accumulates prediction contributions from known leaf ids, mirroring the existing PredictLeaf direction:

• PredictLeaf: features to leaf ids
• PredictFromLeafIds: leaf ids to prediction contributions

GBTree::DoBoost now uses updater-produced node positions and the selected predictor backend to update cached predictions after leaf values are finalized.

Changes

• Add Predictor::PredictFromLeafIds.
• Implement CPU and GPU predictor support.
• Add SYCL predictor delegation to CPU for the new method.
• Remove TreeUpdater::UpdatePredictionCache.
• Remove updater-specific prediction-cache implementations from hist, approx, GPU hist, and SYCL hist updaters.
• Update GBTree training cache updates to use predictor-side leaf-id prediction.
• Extend predictor/cache tests for the new ownership boundary.

Validation

• cmake -S . -B build -DUSE_CUDA=OFF -DGOOGLE_TEST=ON -DUSE_DMLC_GTEST=ON
• cmake --build build --target objxgboost -j2
• cmake --build build --target testxgboost -j2
• ./build/testxgboost --gtest_filter=CpuPredictor.Basic
• ./build/testxgboost --gtest_filter=CPUPredictor.*:CpuPredictor.TrainingPredictionCache:Updater.PredictionCache*:TestPredictionCache.*
• cmake -S . -B build-cuda -DUSE_CUDA=ON -DGOOGLE_TEST=ON -DUSE_DMLC_GTEST=ON
• cmake --build build-cuda --target objxgboost -j2
• cmake --build build-cuda --target testxgboost -j2
• ./build-cuda/testxgboost --gtest_filter=GPUPredictor.Basic:GPUPredictor.PredictLeafBasic:GpuHist.UniformSampling:GpuHist.GradientBasedSampling:TestPredictionCache.GpuHist:TestPredictionCache.GpuMultiHist:TestPredictionCache.GpuApprox

[Copilot]

Pull request overview

This PR moves training prediction-cache accumulation out of individual tree updaters and into the predictor layer by introducing Predictor::PredictFromLeafIds, enabling cache updates from known leaf IDs (node positions) after leaf values are finalized.

Changes:

• Added Predictor::PredictFromLeafIds API and implemented it for CPU and GPU predictors, with SYCL delegating to CPU.
• Updated GBTree::DoBoost to update cached predictions via the selected predictor using updater-produced node positions.
• Removed updater-specific prediction-cache update code paths and updated/extended related unit tests.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/cpp/tree/test_prediction_cache.h	Switches cache-update testing from updater API to PredictFromLeafIds.
tests/cpp/tree/test_gpu_hist.cu	Updates GPU hist tests to use predictor-based cache updates.
tests/cpp/predictor/test_predictor.cc	Adds direct unit coverage for PredictFromLeafIds equivalence vs PredictBatch.
tests/cpp/predictor/test_cpu_predictor.cc	Renames/aligns CPU training cache test naming with new ownership boundary.
src/tree/updater_quantile_hist.cc	Removes quantile-hist updater’s cache-update implementation.
src/tree/updater_gpu_hist.cuh	Removes multi-target GPU hist cache-update implementation.
src/tree/updater_gpu_hist.cu	Removes GPU hist/approx updater cache-update implementations.
src/tree/updater_approx.cc	Removes approx updater cache-update implementation and related include usage.
src/tree/hist/evaluate_splits.h	Removes shared CPU cache-update helpers previously used by updaters.
src/predictor/gpu_predictor.cu	Implements PredictFromLeafIds on GPU using leaf IDs to accumulate predictions.
src/predictor/cpu_predictor.cc	Implements PredictFromLeafIds on CPU using leaf IDs to accumulate predictions.
src/gbm/gbtree.cc	Updates training-time cache updates to use predictor-based leaf-id accumulation.
plugin/sycl/tree/updater_quantile_hist.h	Removes SYCL updater cache-update API and related declarations.
plugin/sycl/tree/updater_quantile_hist.cc	Removes SYCL updater cache-update implementation and registry formatting updates.
plugin/sycl/tree/hist_updater.h	Removes SYCL hist-updater cache-update declaration.
plugin/sycl/tree/hist_updater.cc	Removes SYCL hist-updater cache-update implementation.
plugin/sycl/predictor/predictor.cc	Adds SYCL predictor delegation for PredictFromLeafIds (currently CPU fallback).
include/xgboost/tree_updater.h	Removes TreeUpdater::UpdatePredictionCache from the public interface.
include/xgboost/predictor.h	Adds Predictor::PredictFromLeafIds to the public predictor interface.

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