Clean roberta model & all tokenizers now add special tokens by default (breaking change)
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.
# [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: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/1601?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/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