fix #2399 an ImportError in official example
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2400?src=pr&el=h1) Report
> Merging [#2400](https://codecov.io/gh/huggingface/transformers/pull/2400?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/78528742f169fb9481865aa25726ceca5499e036?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2400/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2400?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2400   +/-   ##
=======================================
  Coverage   73.24%   73.24%           
=======================================
  Files          87       87           
  Lines       14989    14989           
=======================================
  Hits        10979    10979           
  Misses       4010     4010
```



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