fix: use npu_fusion_attention loop (BSND) for prefill_no_cache to fix crash

vllm_npu/attention/attention_v1.py

@@ -449,33 +449,43 @@ class AscendAttentionBackendImpl(AttentionImpl):
         output: torch.Tensor,
         num_tokens: int,
     ) -> torch.Tensor:
-        """Prefill attention without KV cache (self-attention) using _npu_flash_attention."""
+        """Prefill attention without KV cache (self-attention) via per-req loop."""
         import torch_npu  # noqa: F401
 
-        # Huawei uses _npu_flash_attention for prefill
-        # Ensure contiguous inputs
-        query = query.contiguous()
-        key = key.contiguous()
-        value = value.contiguous()
-        
-        # mask needs to be contiguous and cast to expected format if needed
-        # but _npu_flash_attention handles generic mask? 
-        # Huawei code: mask = attn_metadata.attn_mask... 
-        # We'll pass it as is, assuming AscendMetadataBuilder created it correctly.
-        
-        torch_npu._npu_flash_attention(
-            query=query,
-            key=key,
-            value=value,
-            mask=attn_metadata.attn_mask,
-            seq_len=attn_metadata.seq_lens,
-            scale_value=self.scale,
-            num_heads=self.num_heads,
-            num_kv_heads=self.num_kv_heads,
-            out=output
-        )
+        query_start_loc = attn_metadata.query_start_loc
+        seq_lens = attn_metadata.seq_lens
+        num_reqs = len(seq_lens)
 
-        return output[:num_tokens]
+        # Iterate and process each request independently to bypass TND issues
+        for i in range(num_reqs):
+            start = query_start_loc[i].item()
+            end = query_start_loc[i + 1].item()
+            q_len = end - start
+
+            # Extract q, k, v (BSND)
+            q = query[start:end].unsqueeze(0)
+            k = key[start:end].unsqueeze(0)
+            v = value[start:end].unsqueeze(0)
+
+            # Mask (lower triangular for causal)
+            attn_mask = torch.ones(
+                q_len, q_len, dtype=torch.bool, device=query.device
+            ).triu_(diagonal=1).unsqueeze(0)
+
+            # Run npu_fusion_attention (BSND)
+            attn_out = torch_npu.npu_fusion_attention(
+                q, k, v,
+                head_num=self.num_heads,
+                input_layout="BSND",
+                scale=self.scale,
+                atten_mask=attn_mask,
+                pre_tockens=2147483647,
+                next_tockens=0,
+            )
+
+            output[start:end] = attn_out[0]
+
+        return output
 
     # -----------------------------------------------------------------
     # Chunked prefill — mixed prefill+decode via npu_fusion_attention