Fix for fast tokenizers save_pretrained compatibility with Python.
Author: mfuntowicz
Pull request: yes

The name of generated file doesn't match between tokenizers and transformers tokenizers, so transformers is not able to load model saved with tokenizers models.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2933?src=pr&el=h1) Report
> Merging [#2933](https://codecov.io/gh/huggingface/transformers/pull/2933?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6a50d501ec54fd28eed57031ddbba6480768f9bc?src=pr&el=desc) will **decrease** coverage by `1%`.
> The diff coverage is `95%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2933/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2933?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #2933      +/-   ##
=========================================
- Coverage   77.21%   76.2%   -1.01%     
=========================================
  Files          98      98              
  Lines       16030   16040      +10     
=========================================
- Hits        12377   12224     -153     
- Misses       3653    3816     +163
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2933?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/2933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `41.1% <100%> (+1.3%)` | :arrow_up: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `91% <66.66%> (+0.01%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0%> (-81.21%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `85.71% <0%> (-10%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.48% <0%> (-2.3%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `96.03% <0%> (-2.21%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.2% <0%> (-1.35%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `92.54% <0%> (+0.32%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2933?src=pr&el=footer). Last update [6a50d50...f22083c](https://codecov.io/gh/huggingface/transformers/pull/2933?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).