Switch from PreTrainedTokenizerFast to GPT2TokenizerFast and add eos_token & bos_token (#15)

- Switch from PreTrainedTokenizerFast to GPT2TokenizerFast and add eos_token & bos_token (d63779a667c4e0bd0d06f385b407c0c649b36454)

tokenizer_config.json

@@ -1,5 +1,7 @@
 {
   "errors": "replace",
-  "tokenizer_class": "PreTrainedTokenizerFast",
+  "tokenizer_class": "GPT2TokenizerFast",
+  "bos_token": "<|endoftext|>",
+  "eos_token": "<|endoftext|>",
   "model_max_length": 2048
-}
+}