run_finetuning resize token embeddings
Please see #1848 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1849?src=pr&el=h1) Report
> Merging [#1849](https://codecov.io/gh/huggingface/transformers/pull/1849?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0477b307c7501ea76e01b03cb387a2312db752b3?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/1849/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1849?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1849   +/-   ##
=======================================
  Coverage   84.08%   84.08%           
=======================================
  Files          97       97           
  Lines       14316    14316           
=======================================
  Hits        12037    12037           
  Misses       2279     2279
```



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<|||||>Looks good to me, thanks for looking into it!<|||||>You are welcome and thanks @LysandreJik for interested. Do you mind approve and merge, if there is no problem?