Update doc to new model outputs
This is a follow-up from #5438, adapting doc pages and examples.
The other problem is that it downloads all models since it tests all examples. The PR fixes some docstrings so I know some of them were broken.

Side note, the correct command is:
```
RUN_SLOW=yes pytest tests/test_doc_samples.py
```
since they are all marked as slow.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5946?src=pr&el=h1) Report
> Merging [#5946](https://codecov.io/gh/huggingface/transformers/pull/5946?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/604a2355dc61f2888d68aab3adb0c5b648a4f42d&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5946/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5946?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5946      +/-   ##
==========================================
+ Coverage   78.47%   78.51%   +0.03%     
==========================================
  Files         146      146              
  Lines       26214    26214              
==========================================
+ Hits        20572    20582      +10     
+ Misses       5642     5632      -10     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5946?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `81.19% <ø> (ø)` | |
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `82.04% <ø> (ø)` | |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `95.74% <ø> (ø)` | |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.39% <ø> (ø)` | |
| [src/transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kaXN0aWxiZXJ0LnB5) | `97.82% <ø> (ø)` | |
| [src/transformers/modeling\_dpr.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kcHIucHk=) | `97.83% <ø> (ø)` | |
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `81.55% <ø> (ø)` | |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `85.88% <ø> (ø)` | |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `89.21% <ø> (ø)` | |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `89.45% <ø> (ø)` | |
| ... and [4 more](https://codecov.io/gh/huggingface/transformers/pull/5946/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5946?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/5946?src=pr&el=footer). Last update [604a235...766911a](https://codecov.io/gh/huggingface/transformers/pull/5946?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Quite a few of those tests are failing but it appears TensorFlow outputs are unreliable: they are printed at full precision and my guess is that depending on your GPU, you get some different digits starting at 1e-6. Will discuss with @LysandreJik on how to make this more reliable when he's back, in the meantime, merging this one.