[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/ops/fused_moe/prepare_finalize.py

@@ -377,6 +377,15 @@ class PrepareAndFinalizeWithAllGather(PrepareAndFinalize):
             router_logits = self.moe_config.dp_group.all_gather(router_logits, 0)
 
         if prefill_context_parallel_enable() and self.moe_config.pcp_size > 1:
+            forward_context = get_forward_context()
+            max_tokens_across_pcp = forward_context.max_tokens_across_pcp
+
+            self.num_tokens_pcp = hidden_states.shape[0]
+            pad_size = max_tokens_across_pcp - self.num_tokens_pcp
+            if pad_size > 0:
+                hidden_states = nn.functional.pad(hidden_states, (0, 0, 0, pad_size))
+                router_logits = nn.functional.pad(router_logits, (0, 0, 0, pad_size))
+
             hidden_states = get_pcp_group().all_gather(
                 hidden_states,
                 dim=0,