[Bugfix] Move current_platform import to avoid python import cache. (#16601)

Signed-off-by: iwzbi <wzbi@zju.edu.cn>

vllm/attention/selector.py

@@ -14,7 +14,6 @@ import vllm.envs as envs
 from vllm.attention.backends.abstract import AttentionBackend
 from vllm.attention.backends.registry import _Backend, backend_name_to_enum
 from vllm.logger import init_logger
-from vllm.platforms import current_platform
 from vllm.utils import STR_BACKEND_ENV_VAR, resolve_obj_by_qualname
 
 logger = init_logger(__name__)
@@ -192,6 +191,8 @@ def _cached_get_attn_backend(
                 )
 
     # get device-specific attn_backend
+    from vllm.platforms import current_platform
+
     attention_cls = current_platform.get_attn_backend_cls(
         selected_backend,
         head_size,