Fix Seq2SeqTrainer crash when BatchEncoding data is None (#31418)

avoiding crash when BatchEncoding data is None

src/transformers/tokenization_utils_base.py

@@ -800,7 +800,7 @@ class BatchEncoding(UserDict):
         # Otherwise it passes the casts down and casts the LongTensor containing the token idxs
         # into a HalfTensor
         if isinstance(device, str) or is_torch_device(device) or isinstance(device, int):
-            self.data = {k: v.to(device=device) for k, v in self.data.items()}
+            self.data = {k: v.to(device=device) for k, v in self.data.items() if v is not None}
         else:
             logger.warning(f"Attempting to cast a BatchEncoding to type {str(device)}. This is not supported.")
         return self