Share the model view for CPU and GPU.

[trivialfis]

• Extract the device model view and share it with CPU.
• Ensure the model is locked when pulling tree data.
• Use host device vector for storing the tree group.

This PR eliminates another data copy (tree group), and ensures thread safety when the model is trained using GPU but inference is performed with CPU.

[Copilot]

Pull Request Overview

This PR refactors the tree model view infrastructure to enable sharing between CPU and GPU predictors, improving code maintainability and reducing duplication.

Key Changes:

• Introduced GBTreeModelView template class to unify model view creation across CPU and GPU
• Changed tree view constructors to accept DeviceOrd instead of Context*
• Converted tree_info from std::vector<int> to HostDeviceVector<bst_target_t> for device-agnostic access

Reviewed Changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/predictor/gbtree_view.h	New template class for unified tree model views
src/tree/tree_view.h	Updated constructors to accept DeviceOrd instead of Context*
src/tree/tree_view.cc	Refactored to use DeviceOrd for device dispatch
src/predictor/gpu_predictor.cu	Replaced DeviceModel with GBTreeModelView template
src/predictor/cpu_predictor.cc	Adopted GBTreeModelView and unified leaf index logic
src/gbm/gbtree_model.h	Changed tree_info to HostDeviceVector and added TreeGroups() method
src/gbm/gbtree_model.cc	Updated to use HostDeviceVector for tree_info
tests/cpp/predictor/test_predictor.h	Updated test helper to return unique_ptr
tests/cpp/predictor/test_predictor.cc	Updated tests to use new model view interface
tests/cpp/predictor/test_gpu_predictor.cu	Updated GPU tests for new interface
tests/cpp/predictor/test_cpu_predictor.cc	Updated tree view constructor call
src/gbm/gbtree.cc	Moved InitTreesToUpdate implementation and updated tree_info access
src/common/device_vector.cuh	Fixed resize implementation for CTK12.8 compatibility
plugin/sycl/predictor/predictor.cc	Updated to access tree_info through HostDeviceVector
src/data/cat_container.h	Forward declared Json instead of including header
tests/cpp/gbm/test_gbtree.cc	Added clarifying comment

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[trivialfis]

cc @rongou