Fix special tokens addition in decoder #1807
Fixes the issue detailed in #1807

Added special tokens should not be present when decoding with the `skip_special_tokens` flag set to `True`.
Using the `convert_tokens_to_ids` checks whether those added tokens are present and removes them if necessary.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1811?src=pr&el=h1) Report
> Merging [#1811](https://codecov.io/gh/huggingface/transformers/pull/1811?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/155c782a2ccd103cf63ad48a2becd7c76a7d2115?src=pr&el=desc) will **decrease** coverage by `0.08%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1811/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1811?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1811      +/-   ##
==========================================
- Coverage   84.16%   84.08%   -0.09%     
==========================================
  Files          94       94              
  Lines       14185    14047     -138     
==========================================
- Hits        11939    11811     -128     
+ Misses       2246     2236      -10
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1811?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tests/tokenization\_tests\_commons.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl90ZXN0c19jb21tb25zLnB5) | `100% <100%> (ø)` | :arrow_up: |
| [transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rva2VuaXphdGlvbl91dGlscy5weQ==) | `92.12% <100%> (+0.92%)` | :arrow_up: |
| [transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3JvYmVydGEucHk=) | `89.9% <0%> (-0.53%)` | :arrow_down: |
| [transformers/modeling\_tf\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3hsbmV0LnB5) | `87.82% <0%> (-0.37%)` | :arrow_down: |
| [transformers/modeling\_tf\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3RyYW5zZm9feGwucHk=) | `92.21% <0%> (-0.33%)` | :arrow_down: |
| [transformers/tests/modeling\_tf\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2NvbW1vbl90ZXN0LnB5) | `96.8% <0%> (-0.28%)` | :arrow_down: |
| [transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3V0aWxzLnB5) | `92.4% <0%> (-0.28%)` | :arrow_down: |
| [transformers/modeling\_tf\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2N0cmwucHk=) | `97.75% <0%> (-0.11%)` | :arrow_down: |
| [transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3hsbS5weQ==) | `90.39% <0%> (-0.08%)` | :arrow_down: |
| [transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2Rpc3RpbGJlcnQucHk=) | `98.59% <0%> (-0.07%)` | :arrow_down: |
| ... and [6 more](https://codecov.io/gh/huggingface/transformers/pull/1811/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1811?src=pr&el=footer). Last update [155c782...74d0bcb](https://codecov.io/gh/huggingface/transformers/pull/1811?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Nice fix, a lot cleaner as well