adding max_lengths for single sentences and sentences pairs

pytorch_transformers/tokenization_roberta.py

@@ -160,6 +160,14 @@ class RobertaTokenizer(PreTrainedTokenizer):
         text = bytearray([self.byte_decoder[c] for c in text]).decode('utf-8', errors=self.errors)
         return text
 
+    @property
+    def max_len_single_sentence(self):
+        return self.max_len - 2  # take into account special tokens
+
+    @property
+    def max_len_sentences_pair(self):
+        return self.max_len - 4  # take into account special tokens
+
     def add_special_tokens_single_sentence(self, token_ids):
         """
         Adds special tokens to a sequence for sequence classification tasks.