Fix add_special_tokens on fast tokenizers
Fix #4457

By using `flatten`, the following
```
dict_values(['[EOS]', '[BOS]'])
```
was being transformed into this:
```
['[', 'E', 'O', 'S', ']', '[', 'B', 'O', 'S', ']']
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4531?src=pr&el=h1) Report
> Merging [#4531](https://codecov.io/gh/huggingface/transformers/pull/4531?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e19b978151419fe0756ba852b145fccfc96dbeb4&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4531/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4531?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4531      +/-   ##
==========================================
+ Coverage   77.86%   77.88%   +0.01%     
==========================================
  Files         123      123              
  Lines       20566    20570       +4     
==========================================
+ Hits        16014    16020       +6     
+ Misses       4552     4550       -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4531?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4531/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.55% <100.00%> (+0.04%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4531/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.53% <0.00%> (+0.11%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4531/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4531?src=pr&el=footer). Last update [e19b978...afcf0c9](https://codecov.io/gh/huggingface/transformers/pull/4531?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
