Fix CI error about attn backend cls

tpu_commons/platforms/tpu_jax.py

@@ -8,21 +8,23 @@
 from torchax.ops.mappings import j2t_dtype
 from tpu_info import device
 from vllm.inputs import ProcessorInputs, PromptType
-from vllm.platforms.interface import Platform, PlatformEnum, _Backend
+from vllm.platforms.interface import Platform, PlatformEnum
 from vllm.sampling_params import SamplingParams, SamplingType
 
 from tpu_commons.logger import init_logger
 from tpu_commons.models.jax.utils.quantization.quantization_utils import \
     update_vllm_config_for_qwix_quantization
 
 if TYPE_CHECKING:
+    from vllm.attention.backends.registry import _Backend
     from vllm.config import BlockSize, ModelConfig, VllmConfig
     from vllm.pooling_params import PoolingParams
 else:
     BlockSize = None
     ModelConfig = None
     VllmConfig = None
     PoolingParams = None
+    _Backend = None
 
 logger = init_logger(__name__)
 
@@ -51,10 +53,11 @@ class TpuPlatform(Platform):
     ]
 
     @classmethod
-    def get_attn_backend_cls(cls, selected_backend: _Backend, head_size: int,
+    def get_attn_backend_cls(cls, selected_backend: "_Backend", head_size: int,
                              dtype: jnp.dtype, kv_cache_dtype: Optional[str],
                              block_size: int, use_v1: bool, use_mla: bool,
                              has_sink: bool, use_sparse: bool) -> str:
+        from vllm.attention.backends.registry import _Backend
         if selected_backend != _Backend.PALLAS:
             logger.info("Cannot use %s backend on TPU.", selected_backend)