[Attention] Update attention imports (#29540)

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

vllm/config/model.py

@@ -14,6 +14,7 @@ from safetensors.torch import _TYPES as _SAFETENSORS_TO_TORCH_DTYPE
 from transformers.configuration_utils import ALLOWED_LAYER_TYPES
 
 import vllm.envs as envs
+from vllm.attention.backends.registry import AttentionBackendEnum
 from vllm.config.multimodal import MMCacheType, MMEncoderTPMode, MultiModalConfig
 from vllm.config.pooler import PoolerConfig
 from vllm.config.scheduler import RunnerType
@@ -53,7 +54,6 @@ if TYPE_CHECKING:
 
     import vllm.model_executor.layers.quantization as me_quant
     import vllm.model_executor.models as me_models
-    from vllm.attention.backends.registry import AttentionBackendEnum
     from vllm.config.load import LoadConfig
     from vllm.config.parallel import ParallelConfig
     from vllm.model_executor.layers.quantization import QuantizationMethods
@@ -61,7 +61,6 @@ if TYPE_CHECKING:
 else:
     PretrainedConfig = Any
 
-    AttentionBackendEnum = Any
     me_quant = LazyLoader(
         "model_executor", globals(), "vllm.model_executor.layers.quantization"
     )