LongformerTokenizerFast
This PR adds `LongformerTokenizerFast` by sub-classing `RobertaTokenizerFast`.

@patrickvonplaten 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4547?src=pr&el=h1) Report
> Merging [#4547](https://codecov.io/gh/huggingface/transformers/pull/4547?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a34a9896ac2a4a33ff9cd805c76eed914c8d8965&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4547/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4547?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4547   +/-   ##
=======================================
  Coverage   77.87%   77.87%           
=======================================
  Files         123      123           
  Lines       20566    20569    +3     
=======================================
+ Hits        16016    16019    +3     
  Misses       4550     4550           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4547?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/4547/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.13% <100.00%> (ø)` | |
| [src/transformers/tokenization\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4547/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbG9uZ2Zvcm1lci5weQ==) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4547/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4547?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/4547?src=pr&el=footer). Last update [a34a989...453d496](https://codecov.io/gh/huggingface/transformers/pull/4547?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM, but adding @mfuntowicz here, since I'm not very familiar with the FastTokenizers. @mfuntowicz - do we also have to apply changes on the Rust side for this? <|||||>LGTM if the tokenizer doesn't have any different pre-processing / post-processing than the current Roberta Tokenizer 👍 <|||||>Great I think it's good to merge then :-) 