[wip] Allow the device vector to shrink.

[trivialfis]

Need benchmarks for training and prediction.

[Copilot]

Pull request overview

This PR enables the device vector to shrink its allocated memory when resized to significantly smaller sizes, reducing memory consumption. The implementation uses a "dynamic table" approach where the vector only deallocates and shrinks when the new size falls below half the current capacity.

Changes:

• Modified resize() to conditionally shrink capacity when new size is less than half of current capacity
• Updated clear() to immediately deallocate memory and reset capacity to 0
• Added comprehensive tests for both shrinking scenarios (first and second half of dynamic table)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/common/device_vector.cuh	Implements shrinking logic in resize() and immediate deallocation in clear()
tests/cpp/common/test_device_vector.cu	Adds test coverage for shrinking behavior in both halves of the dynamic table and verifies clear() deallocates

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