wrong variable name used
Turned 
`nlp.tokenizer.mask_token` to `fill_mask.tokenizer.mask_token` 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4328?src=pr&el=h1) Report
> Merging [#4328](https://codecov.io/gh/huggingface/transformers/pull/4328?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/241759101e7104192d01a07fc70432fa02ae8cb7&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4328/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4328?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4328   +/-   ##
=======================================
  Coverage   78.18%   78.18%           
=======================================
  Files         120      120           
  Lines       20020    20020           
=======================================
+ Hits        15652    15653    +1     
+ Misses       4368     4367    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4328?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4328/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.08% <0.00%> (+0.12%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4328?src=pr&el=footer). Last update [2417591...f0bb172](https://codecov.io/gh/huggingface/transformers/pull/4328?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
