adapt voxtral (#31095)

Signed-off-by: Patrick von Platen <patrick.v.platen@gmail.com>

vllm/v1/attention/backends/utils.py

@@ -835,6 +835,15 @@ def subclass_attention_backend(
     )
 
 
+def subclass_attention_backend_with_overrides(
+    name_prefix: str,
+    attention_backend_cls: type[AttentionBackend],
+    overrides: dict[str, Any],
+) -> type[AttentionBackend]:
+    name: str = name_prefix + attention_backend_cls.__name__  # type: ignore
+    return type(name, (attention_backend_cls,), overrides)
+
+
 def split_decodes_prefills_and_extends(
     common_attn_metadata: CommonAttentionMetadata,
     decode_threshold: int = 1,