### What this PR does / why we need it?
Fix the incorrect use of the `output` parameter in
`_forward_fia_slidingwindow`:
```
# Original (incorrect)
output, _ = torch_npu.npu_fused_infer_attention_score(...)
output= output.view(batch_size, self.num_heads, self.head_size)
```
In the original writing, the `output `parameter was directly assigned a
new value, which is inconsistent with the interface definition,
resulting in the inability to directly update `output `when calling
externally.
```
attn_output, _ = torch_npu.npu_fused_infer_attention_score(...)
attn_output = attn_output.view(batch_size, self.num_heads, self.head_size)
output[:batch_size] = attn_output[:batch_size]
```
### Does this PR introduce _any_ user-facing change?
No change.
Co-authored-by: GoCHug<gch59135228@163.com>
### How was this patch tested?
vLLM ascend version: v0.13.0rc1
Signed-off-by: acat-rw <892882856@qq.com>
8e66299bf1