### What this PR does / why we need it?
Upstream vLLM PR #30212https://github.com/vllm-project/vllm/pull/30212
refactored the attention backend selection interface, This PR adapts
vllm-ascend's get_attn_backend_cls to align with the new upstream
standard, ensuring compatibility and reducing maintenance overhead.
### Does this PR introduce _any_ user-facing change?
### How was this patch tested?
co-author:[leo-pony][nengjunma@outlook.com](mailto:nengjunma@outlook.com)
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c
---------
Signed-off-by: zxwang <1476209578@qq.com>
Signed-off-by: leo-pony <nengjunma@outlook.com>
Co-authored-by: leo-pony <nengjunma@outlook.com>
b1a853b0f6