Tokenizer fast warnings
- Warning abut padding should not trigger so often now, especially when no padding strategy is provided by the user.

- RoberTa warning is now in RoberTaTokenizer, not GPT2 base class.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2922?src=pr&el=h1) Report
> Merging [#2922](https://codecov.io/gh/huggingface/transformers/pull/2922?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d490b5d5003654f104af3abd0556e598335b5650?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2922/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2922?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2922      +/-   ##
==========================================
- Coverage   75.35%   75.35%   -0.01%     
==========================================
  Files          94       94              
  Lines       15444    15445       +1     
==========================================
  Hits        11638    11638              
- Misses       3806     3807       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2922?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2922/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZ3B0Mi5weQ==) | `96.82% <ø> (-0.03%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2922/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.45% <100%> (-0.14%)` | :arrow_down: |
| [src/transformers/tokenization\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2922/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcm9iZXJ0YS5weQ==) | `100% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2922?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2922?src=pr&el=footer). Last update [d490b5d...6a55286](https://codecov.io/gh/huggingface/transformers/pull/2922?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
