Fixing head masking test
Try to fix the Nan in head masking tests by removing them.
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217?src=pr&el=h1) Report
> Merging [#1217](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/ee027c89f2b8fd0338df39b7e0b48345ea132e99?src=pr&el=desc) will **increase** coverage by `0.38%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #1217      +/-   ##
=========================================
+ Coverage   80.92%   81.3%   +0.38%     
=========================================
  Files          57      57              
  Lines        8014    8018       +4     
=========================================
+ Hits         6485    6519      +34     
+ Misses       1529    1499      -30
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/tests/modeling\_common\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvbW9kZWxpbmdfY29tbW9uX3Rlc3QucHk=) | `73.37% <100%> (+0.17%)` | :arrow_up: |
| [pytorch\_transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX3RyYW5zZm9feGwucHk=) | `34.17% <0%> (+0.18%)` | :arrow_up: |
| [pytorch\_transformers/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX2dwdDIucHk=) | `96.69% <0%> (+0.82%)` | :arrow_up: |
| [pytorch\_transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfdXRpbHMucHk=) | `90.27% <0%> (+1.51%)` | :arrow_up: |
| [...orch\_transformers/tests/tokenization\_utils\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvdG9rZW5pemF0aW9uX3V0aWxzX3Rlc3QucHk=) | `96% <0%> (+4%)` | :arrow_up: |
| [pytorch\_transformers/file\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvZmlsZV91dGlscy5weQ==) | `71.51% <0%> (+4.84%)` | :arrow_up: |
| [...h\_transformers/tests/tokenization\_tests\_commons.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvdG9rZW5pemF0aW9uX3Rlc3RzX2NvbW1vbnMucHk=) | `100% <0%> (+5.98%)` | :arrow_up: |
| [pytorch\_transformers/tokenization\_xlnet.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX3hsbmV0LnB5) | `89.18% <0%> (+7.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217?src=pr&el=footer). Last update [ee027c8...01b9255](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1217?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Superseded by #1203 