[Neuron][Kernel] NKI Flash PagedAttention with BlockSparse Execution Plan

[lingfanyu]

When performing attention computation on RaggedTensor (i.e. a batch of variable-length sequences), we view the batch as one flattened sequence in which original sequences are concatenated along sequence dimension. When performing attention on this flattened sequence, attention that belongs to original sequences happens along the block-diagonal.

Therefore, we have developed a BlockSparse version of flash paged attention to minimize wasted computation. This PR contains two parts:

• An execution planner that takes prompt and context lengths of each request to analyze sparsity and produces an execution plan
• A NKI kernel that accepts the blocksparse plan and performs execution

This PR depends on PR #13245 and #13455 .

@liangfu

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

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 either: Add ready label to the PR or enable auto-merge.

🚀