Automatic testing of examples in documentation
Author: LysandreJik
Pull request: yes

Adds a test that tests the examples in the documentation.
Addsa "Glossary" page for recurring arguments.
Updates the documentation of pytorch & tensorflow models.

Models done:
- [x] ALBERT
- [x] BERT
- [x] GPT-2
- [x] GPT
- [x] Transformer XL
- [x] XLNet
- [x] XLM
- [x] CamemBERT
- [x] RoBERTa
- [x] DistilBERT
- [x] CTRL

To be added (not currently in the docs):
- [x] XLM-RoBERTa

This PR will be merged once these changes have been done. The remaining documentation changes are the following:

- [ ] Update tokenizer documentation
- [ ] Put meaningful examples on each model

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2532?src=pr&el=h1) Report
> Merging [#2532](https://codecov.io/gh/huggingface/transformers/pull/2532?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cefd51c50cc08be8146c1151544495968ce8f2ad?src=pr&el=desc) will **increase** coverage by `0.09%`.
> The diff coverage is `98.59%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2532/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2532?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2532      +/-   ##
==========================================
+ Coverage   74.59%   74.69%   +0.09%     
==========================================
  Files          87       87              
  Lines       14807    14863      +56     
==========================================
+ Hits        11046    11102      +56     
  Misses       3761     3761
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2532?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.82% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `61.09% <ø> (ø)` | :arrow_up: |
| [src/transformers/configuration\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2FsYmVydC5weQ==) | `100% <ø> (ø)` | :arrow_up: |
| [src/transformers/configuration\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3hsbV9yb2JlcnRhLnB5) | `100% <ø> (ø)` | :arrow_up: |
| [src/transformers/configuration\_camembert.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2NhbWVtYmVydC5weQ==) | `100% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `94.27% <100%> (+0.05%)` | :arrow_up: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `87.9% <100%> (+0.17%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `90.41% <100%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_camembert.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jYW1lbWJlcnQucHk=) | `100% <100%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.21% <100%> (+0.3%)` | :arrow_up: |
| ... and [18 more](https://codecov.io/gh/huggingface/transformers/pull/2532/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2532?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/2532?src=pr&el=footer). Last update [cefd51c...904e2b2](https://codecov.io/gh/huggingface/transformers/pull/2532?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).