Fix reference of import in XLM tokenization
Author: shijie-wu
Pull request: yes


Comments:
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190?src=pr&el=h1) Report
> Merging [#1190](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/0287d264e913e10018a95a2723115dc9121e5fc6?src=pr&el=desc) will **decrease** coverage by `0.19%`.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master    #1190     +/-   ##
=========================================
- Coverage   80.85%   80.65%   -0.2%     
=========================================
  Files          46       46             
  Lines        7876     7878      +2     
=========================================
- Hits         6368     6354     -14     
- Misses       1508     1524     +16
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/tokenization\_xlm.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX3hsbS5weQ==) | `82.7% <0%> (-0.71%)` | :arrow_down: |
| [...orch\_transformers/tests/tokenization\_utils\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvdG9rZW5pemF0aW9uX3V0aWxzX3Rlc3QucHk=) | `92% <0%> (-4%)` | :arrow_down: |
| [...h\_transformers/tests/tokenization\_tests\_commons.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvdG9rZW5pemF0aW9uX3Rlc3RzX2NvbW1vbnMucHk=) | `97.16% <0%> (-2.84%)` | :arrow_down: |
| [pytorch\_transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfdXRpbHMucHk=) | `88.61% <0%> (-1.46%)` | :arrow_down: |
| [pytorch\_transformers/file\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvZmlsZV91dGlscy5weQ==) | `70.42% <0%> (-1.41%)` | :arrow_down: |
| [pytorch\_transformers/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX2dwdDIucHk=) | `95.86% <0%> (-0.83%)` | :arrow_down: |
| [pytorch\_transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX3RyYW5zZm9feGwucHk=) | `33.89% <0%> (-0.29%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190?src=pr&el=footer). Last update [0287d26...a15562e](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1190?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Yes, thanks a lot @shijie-wu!