Clean roberta model & all tokenizers now add special tokens by default (breaking change)
Author: LysandreJik
Pull request: yes

The RoBERTa model checks that special tokens are in the input sequence as it cannot function as expected if they are not here. This is not the best practice:

- The print method is not handled on TPU, and the check is problematic when tracing the models
- RoBERTa is the only model to print this warning while other models that require special tokens (BERT, XLNet) don't.

The warning was removed and the encode/encode_plus/prepare_for_model methods now have `add_special_tokens` set to `True` by default. This is a **breaking change**, but it is a better practice.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1601?src=pr&el=h1) Report
> Merging [#1601](https://codecov.io/gh/huggingface/transformers/pull/1601?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/079bfb32fba4f2b39d344ca7af88d79a3ff27c7c?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1601/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1601?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1601      +/-   ##
==========================================
- Coverage    85.9%   85.88%   -0.02%     
==========================================
  Files          91       91              
  Lines       13653    13640      -13     
==========================================
- Hits        11728    11715      -13     
  Misses       1925     1925
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1601?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/1601/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3JvYmVydGEucHk=) | `70.55% <ø> (-0.71%)` | :arrow_down: |
| [transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/1601/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3JvYmVydGEucHk=) | `89.9% <ø> (-0.77%)` | :arrow_down: |
| [transformers/tests/tokenization\_bert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1601/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl9iZXJ0X3Rlc3QucHk=) | `98.66% <100%> (ø)` | :arrow_up: |
| [transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1601/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rva2VuaXphdGlvbl91dGlscy5weQ==) | `91.43% <100%> (ø)` | :arrow_up: |
| [transformers/tests/tokenization\_roberta\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1601/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl9yb2JlcnRhX3Rlc3QucHk=) | `92.45% <100%> (ø)` | :arrow_up: |
| [transformers/tests/tokenization\_xlnet\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1601/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl94bG5ldF90ZXN0LnB5) | `97.91% <100%> (ø)` | :arrow_up: |
| [transformers/tests/tokenization\_tests\_commons.py](https://codecov.io/gh/huggingface/transformers/pull/1601/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl90ZXN0c19jb21tb25zLnB5) | `100% <100%> (ø)` | :arrow_up: |
| [transformers/tests/tokenization\_xlm\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1601/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl94bG1fdGVzdC5weQ==) | `97.72% <100%> (ø)` | :arrow_up: |
| [transformers/tests/tokenization\_distilbert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1601/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl9kaXN0aWxiZXJ0X3Rlc3QucHk=) | `95.23% <100%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1601?src=pr&el=footer). Last update [079bfb3...3617469](https://codecov.io/gh/huggingface/transformers/pull/1601?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Great, LGTM