[python] Fix typehints in Sequence API (#5465)

python-package/lightgbm/basic.py

@@ -1308,15 +1308,19 @@ def _create_sample_indices(self, total_nrow: int) -> np.ndarray:
         assert sample_cnt == actual_sample_cnt.value
         return indices
 
-    def _init_from_ref_dataset(self, total_nrow: int, ref_dataset: 'Dataset') -> 'Dataset':
+    def _init_from_ref_dataset(
+        self,
+        total_nrow: int,
+        ref_dataset: _DatasetHandle
+    ) -> 'Dataset':
         """Create dataset from a reference dataset.
 
         Parameters
         ----------
         total_nrow : int
             Number of rows expected to add to dataset.
-        ref_dataset : Dataset
-            Reference dataset to extract meta from.
+        ref_dataset : object
+            Handle of reference dataset to extract metadata from.
 
         Returns
         -------
@@ -1658,7 +1662,7 @@ def __sample(self, seqs: List[Sequence], total_nrow: int) -> Tuple[List[np.ndarr
     def __init_from_seqs(
         self,
         seqs: List[Sequence],
-        ref_dataset: Optional["Dataset"] = None
+        ref_dataset: Optional[_DatasetHandle] = None
     ) -> "Dataset":
         """
         Initialize data from list of Sequence objects.