Override build_inputs_with_special_tokens for fast tokenizers
Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2912?src=pr&el=h1) Report
> Merging [#2912](https://codecov.io/gh/huggingface/transformers/pull/2912?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/59c23ad9c931ac4fe719abeb3c3851df046ef3a6?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2912/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2912?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2912      +/-   ##
==========================================
+ Coverage    75.3%   75.32%   +0.01%     
==========================================
  Files          94       94              
  Lines       15424    15438      +14     
==========================================
+ Hits        11615    11628      +13     
- Misses       3809     3810       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2912?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2912/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.14% <100%> (+0.05%)` | :arrow_up: |
| [src/transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2912/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydC5weQ==) | `96.99% <100%> (+0.06%)` | :arrow_up: |
| [src/transformers/tokenization\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2912/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcm9iZXJ0YS5weQ==) | `100% <100%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2912/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `61.15% <0%> (-0.18%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2912?src=pr&el=footer). Last update [59c23ad...3b7752b](https://codecov.io/gh/huggingface/transformers/pull/2912?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
