[feature] fia support sliding windows (#5239)

Enable fia to support sliding window function and adapt to the Gemma3
model.

- vLLM version: release/v0.13.0
- vLLM main:
ad32e3e19c
---------
Signed-off-by: nsdie <yeyifan@huawei.com>

vllm_ascend/attention/attention_mask.py

@@ -38,6 +38,7 @@ class AttentionMaskBuilder:
         self.mla_mask = None
         self.chunked_prefill_attn_mask = None
         self.pcp_mla_mask = None
+        self.swa_mask = None
 
     def get_attn_mask(self, max_seq_len: int, dtype: torch.dtype):
         if self.attn_mask_cache is None or max_seq_len > self._seq_len_cached:
@@ -73,3 +74,12 @@ class AttentionMaskBuilder:
             self.pcp_mla_mask = torch.triu(
                 torch.ones(512, 512, device=self.device, dtype=dtype), 1)
         return self.pcp_mla_mask
+
+    def get_swa_mask(self, dtype: torch.dtype, sliding_window):
+        if self.swa_mask is None or self.swa_mask.dtype != dtype:
+            if sliding_window is not None:
+                mask = torch.ones(2048, 2048, dtype=torch.bool)
+                triu_mask = torch.triu(mask, diagonal=1).to(self.device)
+                tril_mask = torch.tril(mask, -sliding_window).to(self.device)
+                self.swa_mask = triu_mask + tril_mask
+        return self.swa_mask