Fix the gpt2 quickstart example
You need to add the SEP (seperator) token to the tokenizer, otherwise the tokenizer.decode will fail with this error:
`ERROR:pytorch_transformers.tokenization_utils:Using sep_token, but it is not set yet.`
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1070?src=pr&el=h1) Report
> Merging [#1070](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1070?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/6f877d9daf36788bad4fd228930939fed6ab12bd?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1070/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1070?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1070   +/-   ##
=======================================
  Coverage   79.61%   79.61%           
=======================================
  Files          42       42           
  Lines        6898     6898           
=======================================
  Hits         5492     5492           
  Misses       1406     1406
```



------

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<|||||>This issue was fixed upstream with #1087
Thanks @oliverguhr 