[Bugfix] Fix Dcp dimension mismatch when enable Mlapo (#4687)

### What this PR does / why we need it?
After enabling Mlapo and DCP, since Mlapo has its own mla_preprocess
logic and does not perform additional all_gather operations on the DCP
group, this will lead to dimension mismatch during the subsequent
forward proces

### Does this PR introduce _any_ user-facing change?

N/A


- vLLM version: v0.12.0
- vLLM main:
vllm-project/vllm@ad32e3e

Signed-off-by: zengran <[email protected]>
Co-authored-by: wangxiyuan <[email protected]>

Author: zengzengran

vllm_ascend/attention/mla_v1.py

@@ -1495,6 +1495,13 @@ def _mla_decode_preprocess(self, hidden_states, kv_cache, attn_metadata):
                                            self.kv_lora_rank)
         decode_q_pe = decode_q_pe.view(bsz, self.num_heads, -1)
 
+        if self.dcp_size > 1:
+            decode_q_no_split = torch.cat([decode_q_nope, decode_q_pe], dim=-1)
+            decode_q_no_split = get_dcp_group().all_gather(
+                decode_q_no_split, 1)
+            decode_q_nope, decode_q_pe = decode_q_no_split.split(
+                [self.kv_lora_rank, self.qk_rope_head_dim], dim=-1)
+
         decode_preprocess_res = DecodeMLAPreprocessResult(
             decode_q_nope, decode_q_pe, decode_k_nope, decode_k_pe)
         return decode_preprocess_res, None