Fix small type hinting error (#7820)

* Fix small type hinting error

* Update tokenization_utils_base.py

* Update src/transformers/tokenization_utils_base.py
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

src/transformers/tokenization_utils_base.py

@@ -602,7 +602,7 @@ class BatchEncoding(UserDict):
         return self
 
     @torch_required
-    def to(self, device: str) -> "BatchEncoding":
+    def to(self, device: Union[str, "torch.device"]) -> "BatchEncoding":
         """
         Send all values to device by calling :obj:`v.to(device)` (PyTorch only).