[Core] Performance optimization for swap_blocks by cuda kernels

[ApostaC]

This PR is part of the big CPU offloading PR #10874 -- this PR contains the new CUDA kernel implementation for swap_blocks.

Performance benchmark

The numbers are collected on A100-40GB-SXM GPUs

# of pages	New implementation	Old implementation
Swap in 1 page	1.21 GB / second	1.21 GB / second
Swap in 1250 pages	12.6 GB / second	3.11 GB / second
Swap out 1 page	1.22 GB / second	1.21 GB / second
Swap out 1250 pages	12.5 GB / second	3.12 GB / seocond

Notes: CUDA graph compatibility

Currently, it pre-allocates a CPU pin-memory tensor for the blocks_to_swap_in and blocks_to_swap_out. It could support CUDA graphs in the future since the address of the pre-allocated buffer won't change.

I did not include it in the PR currently, and as a next step, I can create a new PR for CUDA graphs.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀