Improvements to implementation of str method for datamodule

Corrected the annotation for the internal function and the list that is suppsoed to store the information on the datasets

Author: MrWhatZitToYaa

src/lightning/pytorch/core/datamodule.py

@@ -15,7 +15,7 @@
 
 import inspect
 import os
-from typing import IO, Any, Dict, Iterable, Optional, Union, cast
+from typing import IO, Any, Dict, Iterable, List, Optional, Union, cast
 
 from lightning_utilities import apply_to_collection
 from torch.utils.data import DataLoader, Dataset, IterableDataset
@@ -252,9 +252,9 @@ def __str__(self) -> str:
             A string representation of the datasets that are setup.
 
         """
-        datasets_info: Optional[str] = []
+        datasets_info: Optional[List[str]] = []
 
-        def len_implemented(obj):
+        def len_implemented(obj: Dataset) -> bool:
             try:
                 len(obj)
                 return True