[Attention] MLA with chunked prefill

[LucasWilkinson]

Need to do more benchmarking to see if this makes sense to be on by default in V0, but lays the groundwork for a V1 implementation. (#13111 may help performance)

lm_eval --model vllm --model_args pretrained=deepseek-ai/DeepSeek-V2-Lite-Chat,tensor_parallel_size=2,dtype=auto,gpu_memory_utilization=0.9,trust_remote_code=True,max_model_len=16384,enable_chunked_prefill=False --task gsm8k --num_fewshot=5 --limit 100

vllm (pretrained=deepseek-ai/DeepSeek-V2-Lite-Chat,tensor_parallel_size=2,dtype=auto,gpu_memory_utilization=0.9,trust_remote_code=True,max_model_len=16384,enable_chunked_prefill=False), gen_kwargs: (None), limit: 100.0, num_fewshot: 5, batch_size: 1
|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value|   |Stderr|
|-----|------:|----------------|-----:|-----------|---|----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  | 0.66|±  |0.0476|
|     |       |strict-match    |     5|exact_match|↑  | 0.66|±  |0.0476|


lm_eval --model vllm --model_args pretrained=deepseek-ai/DeepSeek-V2-Lite-Chat,tensor_parallel_size=2,dtype=auto,gpu_memory_utilization=0.9,trust_remote_code=True,max_model_len=16384,enable_chunked_prefill=True --task gsm8k --num_fewshot=5 --limit 100


vllm (pretrained=deepseek-ai/DeepSeek-V2-Lite-Chat,tensor_parallel_size=2,dtype=auto,gpu_memory_utilization=0.9,trust_remote_code=True,max_model_len=16384,enable_chunked_prefill=True), gen_kwargs: (None), limit: 100.0, num_fewshot: 5, batch_size: 1
|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value|   |Stderr|
|-----|------:|----------------|-----:|-----------|---|----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  | 0.66|±  |0.0476|
|     |       |strict-match    |     5|exact_match|↑  | 0.66|±  |0.0476|

Shout-out to @pathorn for assisting with hardening this PR

Future work:

• Allocate the worst case result of self.kv_b_proj(kv_c_normed) in the profile run
• [Attention] MLA with chunked prefill #12639 (comment)
• Improved algo for allocating workspace amongst batch elements
• Improve how the workspace is allocated

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[mergify]

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

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

[mergify]

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

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

[tlrmchlsmth]

ok yeah that makes sense for some of the red tests

[mergify]

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

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

[ZhongYingMatrix]

Hi @LucasWilkinson thx for ur wonderful work!
I am a little confused on the backend that got from get_attn_backend_cls.
Since we should set VLLM_USE_V1 to use chunked prefill, from here, we would get vllm.v1.attention.backends.flash_attn.FlashAttentionBackend instead of vllm.attention.backends.triton_mla.TritonMLABackend?