adding max_lengths for single sentences and sentences pairs

pytorch_transformers/tokenization_bert.py

@@ -139,6 +139,14 @@ class BertTokenizer(PreTrainedTokenizer):
                                                   tokenize_chinese_chars=tokenize_chinese_chars)
         self.wordpiece_tokenizer = WordpieceTokenizer(vocab=self.vocab, unk_token=self.unk_token)
 
+    @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 - 3  # take into account special tokens
+
     @property
     def vocab_size(self):
         return len(self.vocab)