[EM] By pass device copy if the page fits into device memory.

[trivialfis]

No description provided.

[trivialfis]

cc @rongou

[Copilot]

Pull Request Overview

This PR adds logic to skip device copy when an Ellpack page already resides fully in device memory, marks MemCostBytes as [[nodiscard]], and introduces a new test to verify accessor behavior.

• Implement skip-copy branch in EllpackHostCacheStreamImpl::Read
• Add [[nodiscard]] attribute to MemCostBytes declaration and definition
• Add EllpackHostCacheTest.Accessor to cover on-device and host+device scenarios

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/cpp/data/test_extmem_quantile_dmatrix.cu	New test for verifying device-only and hybrid accessors
src/data/ellpack_page_source.cu	Added on_device check to bypass unnecessary copies
src/data/ellpack_page.cuh	Marked MemCostBytes as [[nodiscard]]
src/data/ellpack_page.cu	Updated MemCostBytes definition with [[nodiscard]]

Comments suppressed due to low confidence (1)

src/data/ellpack_page_source.cu:234

• [nitpick] Consider renaming on_device to a more descriptive name like is_fully_on_device_page to clarify that the entire page resides in device memory.

bool on_device = h_page->gidx_buffer.empty() && !d_page->empty();