[Reformer] Add Enwiki8 Reformer Model - Adapt convert script
This PR adapts the reformer conversion script to the newest trax code and adds `reformer-enwik8` to the docs.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4282?src=pr&el=h1) Report
> Merging [#4282](https://codecov.io/gh/huggingface/transformers/pull/4282?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b290c32e1617fa74f44ccd8b83365fe764437be9&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4282/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4282?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4282      +/-   ##
==========================================
+ Coverage   78.39%   78.40%   +0.01%     
==========================================
  Files         120      120              
  Lines       19932    19932              
==========================================
+ Hits        15626    15628       +2     
+ Misses       4306     4304       -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4282?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4282/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.10% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4282/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `74.38% <0.00%> (+0.41%)` | :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/4282?src=pr&el=footer). Last update [b290c32...5babcfc](https://codecov.io/gh/huggingface/transformers/pull/4282?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
