[Feat] support basic pcp&dcp for qwen3next (#6091)

### What this PR does / why we need it?
This PR implements Context Parallelism (CP) support for the Qwen3-Next
model, including PCP (Parallel Context Parallelism) and DCP
(Dynamic/Data Context Parallelism).

- vLLM version: v0.15.0
- vLLM main:
f176443446

---------

Signed-off-by: SunnyLee219 <3294305115@qq.com>
Signed-off-by: Jingchun Gao <gaojingchun1@huawei.com>
Signed-off-by: 白永斌 <baiyongbin3@h-partners.com>
Signed-off-by: Bai Yongbin <845473182@qq.com>
Co-authored-by: SunnyLee219 <3294305115@qq.com>
Co-authored-by: Jingchun Gao <gaojingchun1@huawei.com>
Co-authored-by: 白永斌 <baiyongbin3@h-partners.com>
Co-authored-by: Mengqing Cao <cmq0113@163.com>

vllm_ascend/ascend_forward_context.py

@@ -43,6 +43,7 @@ def set_ascend_forward_context(
     model_instance: torch.nn.Module = None,
     is_draft_model=False,
     skip_compiled: bool = False,
+    max_tokens_across_pcp: int = 0,
     draft_attn_metadatas=None,
 ):
     """A context manager that stores the current forward context,
@@ -139,6 +140,7 @@ def set_ascend_forward_context(
             max_tokens_across_dp = num_tokens
 
         forward_context.max_tokens_across_dp = max_tokens_across_dp
+        forward_context.max_tokens_across_pcp = max_tokens_across_pcp
 
         if num_tokens is not None:
             if num_actual_tokens is None: