### What this PR does / why we need it?
For deepseek v3.2, DSA use FullAttentionSpec, allocate 2 * mla page size
bytes, and we use half of that for k cache in DSA
However, the actual proportion of k cache is not high, which results in
a large amount of kvcache being wasted. The proportion of discarded
kvcache is (576-128)/(576 x 2) = 0.388.
Run the same script to start DeepSeek V3.2 on a single A3 server. The
following shows the comparison of kvcache usage:
Before refactoring
```
[kv_cache_utils.py:1307] GPU KV cache size: 15,872 tokens
```
After refactoring
```
[kv_cache_utils.py:1307] GPU KV cache size: 25,984 tokens
```
This pull request refactors the KV cache allocation for Deepseek v3.2
models that use sparse attention. It replaces the use of
`FullAttentionSpec` with `MLAAttentionSpec` and introduces a more
principled way of calculating KV cache tensor split factors based on
model configuration.
This change removes hardcoded values and correctly sizes the cache
tensors, leading to optimized memory usage and improved code
maintainability.
### Does this PR introduce _any_ user-facing change?
No, this is an internal optimization and does not introduce any
user-facing changes.
### How was this patch tested?
- vLLM version: v0.15.0
- vLLM main:
d7e17aaacd
---------
Signed-off-by: Wang Kunpeng <1289706727@qq.com>
156976b982