More explicit error when failing to tensorize overflowing tokens (#5633)

src/transformers/tokenization_utils_base.py

@@ -511,6 +511,11 @@ class BatchEncoding(UserDict):
 
                 self[key] = tensor
             except:  # noqa E722
+                if key == "overflowing_tokens":
+                    raise ValueError(
+                        "Unable to create tensor returning overflowing tokens of different lengths. "
+                        "Please see if a fast version of this tokenizer is available to have this feature available."
+                    )
                 raise ValueError(
                     "Unable to create tensor, you should probably activate truncation and/or padding "
                     "with 'padding=True' 'truncation=True' to have batched tensors with the same length."