Fix cache default value (#8)

- Fix cache default value (eb8c33d7cd6a1fefa1d977aebd39ae331185ec5d)

configuration_cased.py

@@ -1,4 +1,5 @@
 import os
+from typing import Optional
 
 from transformers.modeling_utils import PretrainedConfig
 
@@ -21,11 +22,11 @@ class CaSEDConfig(PretrainedConfig):
         index_name: str = "cc12m",
         alpha: float = 0.5,
         retrieval_num_results: int = 10,
-        cache_dir: str = os.path.expanduser("~/.cache/cased"),
+        cache_dir: Optional[str] = None,
         **kwargs,
     ):
         super().__init__(**kwargs)
         self.index_name = index_name
         self.alpha = alpha
         self.retrieval_num_results = retrieval_num_results
-        self.cache_dir = cache_dir
+        self.cache_dir = cache_dir or os.path.expanduser("~/.cache/cased")