[Bugfix] fix modelscope snapshot_download serialization (#21536)

Signed-off-by: Andy Xie <andy.xning@gmail.com>

vllm/model_executor/model_loader/default_loader.py

@@ -69,10 +69,10 @@ class DefaultModelLoader(BaseModelLoader):
             # pylint: disable=C.
             from modelscope.hub.snapshot_download import snapshot_download
 
-            if not os.path.exists(model):
             # Use file lock to prevent multiple processes from
             # downloading the same model weights at the same time.
             with get_lock(model, self.load_config.download_dir):
+                if not os.path.exists(model):
                     model_path = snapshot_download(
                         model_id=model,
                         cache_dir=self.load_config.download_dir,