Fix import error in script to convert faisreq roberta checkpoints
Fix ImportError in `convert_roberta_original_pytorch_checkpoint_to_pytorch.py`, see #1459.

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1512?src=pr&el=h1) Report
> Merging [#1512](https://codecov.io/gh/huggingface/transformers/pull/1512?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a701c9b32126f1e6974d9fcb3a5c3700527d8559?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/1512/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1512?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1512   +/-   ##
=======================================
  Coverage   85.98%   85.98%           
=======================================
  Files          91       91           
  Lines       13579    13579           
=======================================
  Hits        11676    11676           
  Misses       1903     1903
```



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<|||||>Looks good to me, thanks!