[Kernel] Add prefix-caching support for phi-3-small-8k/128k model triton kernel

[congcongchen123]

This PR is a feature enrichment of https://github.com/vllm-project/vllm/pull/4799 which introduces blocksparse flash attention, Microsoft Phi-3-Small-8K and Phi-3-Small-128K models.

This PR modifies the block-sparse attention prefill Triton kernel to add prefix-caching support.

• Tested utilizing tool: benchmarks/benchmark_prefix_caching.py --model=microsoft/Phi-3-small-8k-instruct --enable-prefix-caching with different prompts, and verified that the output is correct.
• Added unit test.
• Benchmark phi3-small 8k/128k model with prefix-caching enabled, and verified that it has achieved significant TTFT latency gain with different prefix-cache hit rates.

Thanks for offline discussion @linxihui, @wschin !

FIX #xxxx (link existing issues this PR will resolve)

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[wschin]

@comaniac, would you mind find someone to review this PR? This is an important model. If reviewer feels reviewing kernel takes too long, maybe requesting more numerical tests and profiling result? Many thanks.

[comaniac]

@comaniac, would you mind find someone to review this PR? This is an important model. If reviewer feels reviewing kernel takes too long, maybe requesting more numerical tests and profiling result? Many thanks.

I'll review it ASAP. @mgoin @tlrmchlsmth @pcmoritz could any of you also help review the kernel implementation? Meanwhile, numerical tests are definitely required, and do you need to tune the kernel configs offline like we did for the MoE kernels?

[mergify[bot]]

This pull request has merge conflicts that must be resolved before it can be merged. Please rebase the PR, @congcongchen123.

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

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