fixed mypy warnings for files vllm/v1/attention with TEMPORARY workaround (#31465)

Signed-off-by: Zhuohao Yang <zy242@cornell.edu>
Co-authored-by: Zhuohao Yang <zy242@cornell.edu>
Co-authored-by: Wentao Ye <44945378+yewentao256@users.noreply.github.com>

vllm/model_executor/layers/attention_layer_base.py

@@ -4,7 +4,7 @@
 
 from abc import ABC, abstractmethod
 
-from vllm.attention.backends.abstract import AttentionBackend
+from vllm.attention.backends.abstract import AttentionBackend, AttentionImpl
 from vllm.config import VllmConfig
 from vllm.v1.kv_cache_interface import KVCacheSpec
 
@@ -18,6 +18,8 @@ class AttentionLayerBase(ABC):
     from different layer types.
     """
 
+    impl: "AttentionImpl"
+
     @abstractmethod
     def get_attn_backend(self) -> type[AttentionBackend]:
         """Get the attention backend class for this layer."""