[Attention] Update attention imports (#29540)

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

vllm/config/multimodal.py

@@ -2,19 +2,15 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
 from collections.abc import Mapping
-from typing import TYPE_CHECKING, Any, Literal, TypeAlias
+from typing import Any, Literal, TypeAlias
 
 from pydantic import ConfigDict, Field, field_validator, model_validator
 from pydantic.dataclasses import dataclass
 
+from vllm.attention.backends.registry import AttentionBackendEnum
 from vllm.config.utils import config
 from vllm.utils.hashing import safe_hash
 
-if TYPE_CHECKING:
-    from vllm.attention.backends.registry import AttentionBackendEnum
-else:
-    AttentionBackendEnum = Any
-
 
 @dataclass
 class BaseDummyOptions:
@@ -170,9 +166,6 @@ class MultiModalConfig:
     def _validate_mm_encoder_attn_backend(
         cls, value: str | AttentionBackendEnum | None
     ) -> AttentionBackendEnum | None:
-        # We need to import the real type here (deferred to avoid circular import).
-        from vllm.attention.backends.registry import AttentionBackendEnum
-
         if isinstance(value, str) and value.upper() == "XFORMERS":
             raise ValueError(
                 "Attention backend 'XFORMERS' has been removed (See PR #29262 for "