rm the default value of dataset

csst_dfs_client/plan.py

 import os
 from typing import Optional, IO, Tuple, Literal, Union
-from .common import request, Result, constants
+from .common import request, Result
 DateTimeTuple = Tuple[str, str]
 
 def find(mode: Optional[str] = None,
@@ -11,7 +11,7 @@ def find(mode: Optional[str] = None,
         obs_type: Optional[str] = None,
         object_name: Optional[str] = None,
         obstime: Optional[DateTimeTuple] = None,
-        dataset: str = constants.DEFAULT_DATASET,
+        dataset: Optional[str] = None,
         page: int = 1,
         limit: int = 0) -> Result:
     """