add unk_token to gpt2

pytorch_transformers/tokenization_gpt2.py

@@ -102,7 +102,7 @@ class GPT2Tokenizer(PreTrainedTokenizer):
     pretrained_vocab_files_map = PRETRAINED_VOCAB_FILES_MAP
     max_model_input_sizes = PRETRAINED_POSITIONAL_EMBEDDINGS_SIZES
 
-    def __init__(self, vocab_file, merges_file, errors='replace',
+    def __init__(self, vocab_file, merges_file, errors='replace', unk_token="<|endoftext|>",
                  bos_token="<|endoftext|>", eos_token="<|endoftext|>", **kwargs):
         super(GPT2Tokenizer, self).__init__(bos_token=bos_token, eos_token=eos_token, **kwargs)
 
@@ -177,9 +177,7 @@ class GPT2Tokenizer(PreTrainedTokenizer):
 
     def _convert_token_to_id(self, token):
         """ Converts a token (str/unicode) in an id using the vocab. """
-        if token in self.encoder:
-            return self.encoder.get(token)
-        return self.encoder.get(self.unk_token)
+        return self.encoder.get(token, self.encoder.get(self.unk_token))
 
     def _convert_id_to_token(self, index):
         """Converts an index (integer) in a token (string/unicode) using the vocab."""