[Attention] Move Backend enum into registry (#25893)

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

vllm/model_executor/models/siglip2navit.py

@@ -13,6 +13,7 @@ from torch.nn import functional as F
 from transformers import Siglip2VisionConfig
 from transformers.configuration_utils import PretrainedConfig
 
+from vllm.attention.backends.registry import _Backend
 from vllm.attention.layer import check_upstream_fa_availability
 from vllm.distributed import divide, get_tensor_model_parallel_world_size
 from vllm.model_executor.layers.activation import get_act_fn
@@ -22,7 +23,6 @@ from vllm.model_executor.layers.linear import (ColumnParallelLinear,
                                                RowParallelLinear)
 from vllm.model_executor.layers.quantization import QuantizationConfig
 from vllm.model_executor.model_loader.weight_utils import default_weight_loader
-from vllm.platforms import _Backend
 
 from .vision import get_vit_attn_backend