[Feat] Adapted mtp function to Qwen3-next (#3918)

### What this PR does / why we need it?

Adapts mtp function to Qwen3-next.

- vLLM version: v0.11.0
- vLLM main:
83f478bb19

Signed-off-by: drslark <slarksblood@qq.com>

vllm_ascend/attention/attention_v1.py

@@ -252,6 +252,17 @@ class AscendAttentionMetadataBuilder:
         self.dcp_rank = get_decode_context_model_parallel_rank(
         ) if self.dcp_size > 1 else 0
 
+        self.speculative_config = vllm_config.speculative_config
+        self.decode_threshold = 1
+        if self.speculative_config:
+            spec_token_num = self.speculative_config.num_speculative_tokens
+            self.decode_threshold += spec_token_num
+            assert self.decode_threshold <= 16, f"decode_threshold exceeded \
+                npu_fused_infer_attention_score TND layout's limit of 16, \
+                got {self.decode_threshold}"
+
+        AscendAttentionMetadataBuilder.reorder_batch_threshold = self.decode_threshold
+
     def reorder_batch(self, input_batch,
                       scheduler_output: "SchedulerOutput") -> bool:
         return False