modelscope: fix issue when model parameter is not a model id but path of the model. (#2489)

vllm/config.py

@@ -93,9 +93,12 @@ class ModelConfig:
             # download model from ModelScope hub,
             # lazy import so that modelscope is not required for normal use.
             from modelscope.hub.snapshot_download import snapshot_download  # pylint: disable=C
+            if not os.path.exists(model):
                 model_path = snapshot_download(model_id=model,
                                                cache_dir=download_dir,
                                                revision=revision)
+            else:
+                model_path = model
             self.model = model_path
             self.download_dir = model_path
             self.tokenizer = model_path