### What this PR does / why we need it?
kvpool decode save kvcache
now only support mla
### Does this PR introduce _any_ user-facing change?
### How was this patch tested?
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c
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Signed-off-by: baxingpiaochong <771405853@qq.com>
Co-authored-by: Chao Lei <leichao139636@163.com>