[Bugfix] Fix Mamba multistep

[tlrmchlsmth]

Implements advance_step for PlaceholderAttention so that Mamba and other attention-free models function with multistep scheduling.

Fixes the following error in buildkite

ValueError: Multi-Step not supported for attention backend: NO_ATTENTION. Set VLLM_ATTENTION_BACKEND to a value from [‘FLASH_ATTN’, ‘ROCM_FLASH’, ‘FLASHINFER’].

[github-actions]

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

[youkaichao]

thanks for the fix! please test locally if it passes.

[tlrmchlsmth]

@youkaichao Of course I tested it locally! 😆