[bugfix] Fix MHA model runtime error in aclgraph mode (#5397)

### What this PR does / why we need it?
Currently, MHA models (eg: minicpm-2b, Baichuan-7b) will encounter
errors when running in piecewise graph mode, with error messages similar
to:
```
(E89999):  When layout is TND and PA not enabled, keyT(8) and valueT(8) must be equal to the last element of actualSeqenceLengthKV(5)[FUNC:CheckInputShapeWhenLayoutIsTND][FILE:prompt_flash_attention_tiling.cpp][LINE:3618]
```
The error occurs because the qkv in the Prefill stage is also padded,
causing the shape to be inconsistent with actual_seq_lengths.
Add unpadding logic for kv.

- vLLM version: release/v0.13.0
- vLLM main:
254f6b9867

Signed-off-by: Wang Kunpeng <1289706727@qq.com>

vllm_ascend/attention/attention_v1.py

@@ -532,6 +532,9 @@ class AscendAttentionBackendImpl(AttentionImpl):
             = self._get_fia_params(key, value, attn_metadata)
         num_tokens = attn_metadata.actual_seq_lengths_q[-1]
         query = query[:num_tokens]
+        if attn_metadata.attn_state == AscendAttentionState.PrefillNoCache:
+            key = key[:num_tokens]
+            value = value[:num_tokens]
         # Get workspace from cache or calculate it if not present.
         attn_output, _ = torch_npu.npu_fused_infer_attention_score(
             query=query,