[300I] support decode-only aclgraph mode (#6849)

### What this PR does / why we need it?
310p aclgraph mode, but has some problems:
- the event-id hardware limit, the num of graph will be limited.
- the cann version support this feature cannot be get from external of
huawei.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
local test
- vLLM version: v0.15.0
- vLLM main:
83b47f67b1

Signed-off-by: Tflowers-0129 <2906339855@qq.com>

vllm_ascend/_310p/attention/attention_v1.py

@@ -110,7 +110,19 @@ class AscendAttentionBackendImpl310(AscendAttentionBackendImpl):
                 device=query.device,
                 non_blocking=True,
             )
-        return super().forward_paged_attention(query, attn_metadata, output)
+
+        torch_npu._npu_paged_attention(
+            query=query,
+            key_cache=self.key_cache,
+            value_cache=self.value_cache,
+            num_kv_heads=self.num_kv_heads,
+            num_heads=self.num_heads,
+            scale_value=self.scale,
+            block_table=attn_metadata.block_tables,
+            context_lens=attn_metadata.seq_lens,
+            out=output,
+        )
+        return output
 
     def forward_prefill_310(self, query, key, value, attn_metadata, output):
         """