[CI Failure] Fix backend selection for encoder-only models (#28534)

Signed-off-by: Huamin Li <3ericli@gmail.com>

vllm/attention/selector.py

@@ -76,6 +76,7 @@ def get_attn_backend(
     use_mla: bool = False,
     has_sink: bool = False,
     use_sparse: bool = False,
+    attn_type: str | None = None,
 ) -> type[AttentionBackend]:
     """Selects which attention backend to use and lazily imports it."""
 
@@ -94,6 +95,7 @@ def get_attn_backend(
         use_mla=use_mla,
         has_sink=has_sink,
         use_sparse=use_sparse,
+        attn_type=attn_type,
     )
 
 
@@ -106,6 +108,7 @@ def _cached_get_attn_backend(
     use_mla: bool = False,
     has_sink: bool = False,
     use_sparse: bool = False,
+    attn_type: str | None = None,
 ) -> type[AttentionBackend]:
     # Check whether a particular choice of backend was
     # previously forced.
@@ -159,6 +162,7 @@ def _cached_get_attn_backend(
             use_mla,
             has_sink,
             use_sparse,
+            attn_type,
         )
     else:
         attention_cls = current_platform.get_attn_backend_cls(
@@ -170,6 +174,7 @@ def _cached_get_attn_backend(
             use_mla,
             has_sink,
             use_sparse,
+            attn_type,
         )
     if not attention_cls:
         raise ValueError(