added tokenizers serialization tests

tests/tokenization_openai_test.py

@@ -52,5 +52,21 @@ class OpenAIGPTTokenizationTest(unittest.TestCase):
         self.assertListEqual(
             tokenizer.convert_tokens_to_ids(input_tokens), input_bpe_tokens)
 
+        vocab_file, merges_file = tokenizer.save_vocabulary(vocab_path="/tmp/")
+        tokenizer.from_pretrained("/tmp/")
+        os.remove(vocab_file)
+        os.remove(merges_file)
+
+        text = "lower"
+        bpe_tokens = ["low", "er</w>"]
+        tokens = tokenizer.tokenize(text)
+        self.assertListEqual(tokens, bpe_tokens)
+
+        input_tokens = tokens + ["<unk>"]
+        input_bpe_tokens = [14, 15, 20]
+        self.assertListEqual(
+            tokenizer.convert_tokens_to_ids(input_tokens), input_bpe_tokens)
+
+
 if __name__ == '__main__':
     unittest.main()