[Refactor][WIP] Refactor mla_v1 by moving all MLA preprocessing ops into mla_v1 attention impl (#2465)

### What this PR does / why we need it?
In order to support fused kernels, multi-stream, communication
optimization etc, it's better to aggregate all opreations in Attention
layer togather. This PR tries to refactor mla_v1 by moving all MLA
preprocessing ops into mla_v1 attention impl.
Note that new mla_v1 doesn't take torchair into consideration. So this
PR can only be merged after torchair related mla_v1 is isolated into a
new file.
### Does this PR introduce _any_ user-facing change?
NO
### How was this patch tested?

### Features Test

<img width="506" height="141" alt="image"
src="https://github.com/user-attachments/assets/f1ab2906-a1ac-4450-8433-94811cd89466"
/>

### Performance After Refact
<img width="648" height="486" alt="image"
src="https://github.com/user-attachments/assets/e33e038c-c5d9-4ba7-a8e9-1ac22f9833eb"
/>

### Performance Before Refact
<img width="618" height="494" alt="image"
src="https://github.com/user-attachments/assets/83861dc2-dc51-4af3-9310-90ab10c43bb1"
/>


- vLLM version: v0.10.1.1
- vLLM main:
e03940762b

---------

Signed-off-by: lwq <liwenquan5@huawei.com>
Signed-off-by: whx-sjtu <2952154980@qq.com>
Signed-off-by: SunnyLee219 <3294305115@qq.com>
Co-authored-by: lwq <liwenquan5@huawei.com>
Co-authored-by: whx-sjtu <2952154980@qq.com>

docs/source/user_guide/configuration/additional_config.md

@@ -31,6 +31,7 @@ The following table lists the additional configuration options available in vLLM
 | `refresh`                     | bool | `false` | Whether to refresh global ascend config content. This value is usually used by rlhf or ut/e2e test case.     |
 | `expert_map_path`             | str  | `None` | When using expert load balancing for the MOE model, an expert map path needs to be passed in. |
 | `chunked_prefill_for_mla`     | bool | `False` | Whether to enable the fused operator-like chunked_prefill. |
+| `enable_prefetch`     | bool | `False` | Whether to enable weight prefetch. |
 | `kv_cache_dtype`     | str | `None` | When using the kv cache quantization method, kv cache dtype needs to be set, currently only int8 is supported. |
 | `enable_shared_expert_dp`     | bool | `False` | When the shared expert in DP, it has better performance but consumes more memory. Currently only DeepSeek series models are supported to use. |