[6/N][Attention] Move utils to more appropriate locations (#32215)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

vllm/v1/attention/backend.py

@@ -4,7 +4,7 @@
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
 from enum import Enum
-from typing import TYPE_CHECKING, ClassVar, Generic, Protocol, TypeVar, get_args
+from typing import TYPE_CHECKING, Any, ClassVar, Generic, Protocol, TypeVar, get_args
 
 import numpy as np
 import torch
@@ -734,3 +734,27 @@ class MLAAttentionImpl(AttentionImpl[T], Generic[T]):
 
 def is_quantized_kv_cache(kv_cache_dtype: str) -> bool:
     return kv_cache_dtype != "auto"
+
+
+def subclass_attention_backend(
+    name_prefix: str,
+    attention_backend_cls: type[AttentionBackend],
+    builder_cls: type[AttentionMetadataBuilder[M]],
+) -> type[AttentionBackend]:
+    """
+    Return a new subclass where `get_builder_cls` returns `builder_cls`.
+    """
+    name: str = name_prefix + attention_backend_cls.__name__  # type: ignore
+
+    return type(
+        name, (attention_backend_cls,), {"get_builder_cls": lambda: builder_cls}
+    )
+
+
+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)