[CUDA] Enable full cudagraph for FlashMLA

[ProExpertProg]

Enable fullgraph CUDAGraph capture for the FlashMLA decode case.

Hacks:

• building the capture metadata
• prefill batch bypasses compiled code and manually calls eager code

Tested with:

python examples/offline_inference/basic/generate.py --model deepseek-ai/DeepSeek-V2-Lite --trust-remote-code -O {"full_cuda_graph":true}

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[hypnopump]

Could you give some details on speedup associated with this modification?

[mergify]

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

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

[ProExpertProg]

I'm thinking about setting a context to bypass Inductor backend directly here, but I guess we could also separately compile (and capture) prefill stage - any thoughts?
cc @youkaichao