Can't find gpt2 vocab file. 
When I run this 
```
tokenizer = GPT2Tokenizer.from_pretrained(pretrained_model_name_or_path='gpt2',cache_dir=None)
```
I am getting this 
```
Model name 'gpt2' was not found in model name list (gpt2). We assumed 'gpt2' was a path or url but couldn't find files https://s3.amazonaws.com/models.huggingface.co/bert/gpt2-vocab.json and https://s3.amazonaws.com/models.huggingface.co/bert/gpt2-merges.txt at this path or url.
```

Can anyone tell me where I am doing wrong. 
I got the solution.
