use correct ckpt path when base_dir not available (#4101)

* base_dir may not present all time and results in incorrect path

* Update replace_module.py

* Update config.py

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Co-authored-by: NMichael Wyatt <michaelwyatt@microsoft.com>

deepspeed/inference/config.py

@@ -202,7 +202,7 @@ class DeepSpeedInferenceConfig(DeepSpeedConfigModel):
     Path to deepspeed compatible checkpoint or path to JSON with load policy.
     """
 
-    base_dir: str = None
+    base_dir: str = ""
     """
     This shows the root directory under which all the checkpoint files exists.
     This can be passed through the json config too.