[Attention] Temporarily add back pa for small batch sizes. (#4765)

### What this PR does / why we need it?
This PR adds back pa in scenarios of small batch sizes due to
performance consideration. Will remove pa once fia performs better than
pa in all scenarios.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
CI passed with existing test.


- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

Signed-off-by: whx-sjtu <2952154980@qq.com>
Co-authored-by: weijinqian0 <1184188277@qq.com>

vllm_ascend/attention/utils.py

@@ -1,13 +1,29 @@
 from dataclasses import dataclass
+from functools import lru_cache
 from typing import Any, List, Optional
 
 import torch
 import torch.nn.functional as F
+from vllm.config import get_current_vllm_config
 from vllm.distributed.kv_transfer import (get_kv_transfer_group,
                                           has_kv_transfer_group,
                                           is_v1_kv_transfer_group)
 from vllm.forward_context import ForwardContext, get_forward_context
 
+from vllm_ascend.utils import get_ascend_config
+
+
+@lru_cache
+def using_paged_attention(runtime_shape: int) -> bool:
+    vllm_config = get_current_vllm_config()
+    if vllm_config.speculative_config is not None:
+        return False
+    from vllm.config.compilation import CUDAGraphMode
+    if vllm_config.compilation_config.cudagraph_mode != CUDAGraphMode.FULL_DECODE_ONLY:
+        return False
+
+    return runtime_shape in get_ascend_config().pa_shape_list
+
 
 @dataclass
 # class AscendCommonLongSequenceMetadata: