No silent error when d_head already in the configuration
closes #4696
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4747?src=pr&el=h1) Report
> Merging [#4747](https://codecov.io/gh/huggingface/transformers/pull/4747?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ed4df85572924871758ca32133b46116121c706f&el=desc) will **increase** coverage by `0.09%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4747/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4747?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4747      +/-   ##
==========================================
+ Coverage   77.12%   77.22%   +0.09%     
==========================================
  Files         128      128              
  Lines       21061    21063       +2     
==========================================
+ Hits        16243    16265      +22     
+ Misses       4818     4798      -20     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4747?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/4747/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3hsbmV0LnB5) | `94.00% <100.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4747/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4747/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.59% <0.00%> (+0.11%)` | :arrow_up: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4747/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `35.03% <0.00%> (+6.36%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4747?src=pr&el=footer). Last update [ed4df85...fe85f3e](https://codecov.io/gh/huggingface/transformers/pull/4747?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
