Implement double-buffer for ellpack.

[trivialfis]

This helps us to have the external memory cache in two different places.

• Make the accessor a variant type.
• Most of the changes are for calling a visit function instead of actual code changes.

todos:

• Test the new iterator.
• Test for external memory.
• Alignment.
• Profiling.

[Copilot]

Pull Request Overview

This PR implements a double-buffer design for Ellpack pages so that external memory can leverage distinct host and device caches with a variant‐based accessor. Key changes include refactoring all direct accessor calls to use a visitor pattern (via Visit/VisitOnHost), updating test files to call GetHostEllpack/GetDeviceEllpack, and introducing a double buffer accessor (DoubleEllpackAccessor).

Reviewed Changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/cpp/*	Updated testing code to use Visit and GetHostEllpack
src/data/ellpack_page.cuh & related files	Introduced variant-based accessor and double-buffer logic
src/tree/updater_gpu_hist.cu	Updated accessor calls via lambdas using the new API
src/common/compressed_iterator.h	Extended compressed iterator to support double buffering
include/xgboost/base.h	Updated copyright years and minor style adjustments

Comments suppressed due to low confidence (2)

src/data/ellpack_page.cuh:325

• [nitpick] Consider adding more detailed documentation for the Visit and VisitOnHost methods to clarify the expected callback signature and how both accessor variants are handled.

template <typename Fn> decltype(auto) Visit(Context const* ctx, common::Span<FeatureType const> feature_types, Fn&& fn) const {

src/data/ellpack_page_source.cu:249

• There appears to be an extra semicolon after the call to dst->NullValue(), which can be removed for cleaner code.

auto null = dst->NullValue();;

[trivialfis]

cc @rongou