BartTokenizerFast
This PR adds `BartTokenizerFast` by subclassing `RobertaTokenizerFast`

@sshleifer @mfuntowicz 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4878?src=pr&el=h1) Report
> Merging [#4878](https://codecov.io/gh/huggingface/transformers/pull/4878?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/86578bb04c9b34f9d8e35cd4fad42a85910dd9e9&el=desc) will **decrease** coverage by `0.38%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4878/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4878?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4878      +/-   ##
==========================================
- Coverage   77.55%   77.16%   -0.39%     
==========================================
  Files         128      128              
  Lines       21791    21794       +3     
==========================================
- Hits        16899    16818      -81     
- Misses       4892     4976      +84     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4878?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/4878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.13% <100.00%> (ø)` | |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/4878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.12% <100.00%> (+0.38%)` | :arrow_up: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (-28.03%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.61% <0.00%> (+0.11%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4878/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.73% <0.00%> (+0.46%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4878?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4878?src=pr&el=footer). Last update [86578bb...1752816](https://codecov.io/gh/huggingface/transformers/pull/4878?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>I think we will need a test, there are no tests for `RobertaTokenizerFast` in `test_tokenization_roberta.py` file.<|||||>The test is in `test_tokenization_fast`, so I think we're OK on that front.
Going to wait for @n1t0 or @mfuntowicz to approve and merge, because they are working on this concurrently. 