fix labels (#6213)

src/transformers/data/data_collator.py

@@ -87,6 +87,7 @@ class DataCollatorForLanguageModeling:
             return {"input_ids": inputs, "labels": labels}
         else:
             labels = batch.clone().detach()
+            if self.tokenizer.pad_token_id is not None:
                 labels[labels == self.tokenizer.pad_token_id] = -100
             return {"input_ids": batch, "labels": labels}