[LongformerTokenizerFast] add LongformerTokenizerFast in AutoTokenizer
This PR adds LongformerTokenizerFast in AutoTokenizer. Fixes #6459

@patrickvonplaten 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6463?src=pr&el=h1) Report
> Merging [#6463](https://codecov.io/gh/huggingface/transformers/pull/6463?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/54c687e97c92efe6eba9e537bd98b47d9005a279&el=desc) will **increase** coverage by `0.18%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6463/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6463?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6463      +/-   ##
==========================================
+ Coverage   79.91%   80.09%   +0.18%     
==========================================
  Files         153      153              
  Lines       28005    28005              
==========================================
+ Hits        22379    22431      +52     
+ Misses       5626     5574      -52     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6463?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/6463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `97.72% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.53% <0.00%> (-63.20%)` | :arrow_down: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.42% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.44% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `96.58% <0.00%> (+27.51%)` | :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/6463?src=pr&el=footer). Last update [54c687e...7f1278b](https://codecov.io/gh/huggingface/transformers/pull/6463?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
