[Longformer] more models + model cards
This PR adds the following:

- Longformer models trained with frozen-roberta weights

- Model cards

- Model names start with `allenai/`

- Remove unnecessary type casting

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4628?src=pr&el=h1) Report
> Merging [#4628](https://codecov.io/gh/huggingface/transformers/pull/4628?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a801c7fd74f56a651ba43bfc93eba93c63e84766&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4628/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4628?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4628      +/-   ##
==========================================
- Coverage   78.02%   78.02%   -0.01%     
==========================================
  Files         124      124              
  Lines       20626    20625       -1     
==========================================
- Hits        16094    16093       -1     
  Misses       4532     4532              
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4628?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4628/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2xvbmdmb3JtZXIucHk=) | `100.00% <ø> (ø)` | |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4628/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `97.40% <100.00%> (-0.01%)` | :arrow_down: |
| [src/transformers/tokenization\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4628/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbG9uZ2Zvcm1lci5weQ==) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4628/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.41% <0.00%> (-0.12%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4628/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (+0.16%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4628?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/4628?src=pr&el=footer). Last update [a801c7f...06e80ad](https://codecov.io/gh/huggingface/transformers/pull/4628?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
