[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

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This pull request has merge conflicts that must be resolved before it can be merged. Please rebase the PR, @lingfanyu.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork