[Bugfix] Fix model run _npu_flash_attention hang issue (#4410)

Fix model run _npu_flash_attention in _forward_prefill_no_cache hang
issue, it was caused by wrong attention mask dtype.
### How was this patch tested?
Yes, tesed on Qwen2.5-VL and Qwen2.5-Omni

- vLLM version: v0.11.0
- vLLM main:
2918c1b49c

Signed-off-by: Ting FU <futing10@huawei.com>

vllm_ascend/worker/model_runner_v1.py

@@ -991,8 +991,8 @@ class NPUModelRunner(LoRAModelRunnerMixin):
                 max_seq_len, self.dtype, self.device)
         # Prefill with cache hit.
         elif attn_state == AscendAttentionState.PrefillCacheHit:
-            return self.attn_mask_builder.get_attn_mask(
-                2048, self.dtype, self.device)
+            return self.attn_mask_builder.get_splitfuse_attn_mask().to(
+                torch.bool)
         # Decode-only situation.
         else:
             return None