adding max_lengths for single sentences and sentences pairs

pytorch_transformers/tokenization_utils.py

@@ -67,6 +67,14 @@ class PreTrainedTokenizer(object):
                                  "pad_token", "cls_token", "mask_token",
                                  "additional_special_tokens"]
 
+    @property
+    def max_len_single_sentence(self):
+        return self.max_len  # Default to max_len but can be smaller in specific tokenizers to take into account special tokens
+
+    @property
+    def max_len_sentences_pair(self):
+        return self.max_len  # Default to max_len but can be smaller in specific tokenizers to take into account special tokens
+
     @property
     def bos_token(self):
         """ Beginning of sentence token (string). Log an error if used while not having been set. """