Add a few of typos corrections, bugs fixes and small improvements
Author: thomwolf
Pull request: yes

- Add a `force_download` option to `from_pretrained` methods to override a corrupted file.
- Add a `proxies` option to `from_pretrained` methods to be able to use proxies.
- Update models doc (superseded #984)
- Fix a small bug when using Bert's `save_vocabulary` method with the path to a file instead of a directory (#1014)
- Detailed doc strings following #808 
- Detailed doc strings following #1034 

Comments:
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057?src=pr&el=h1) Report
> Merging [#1057](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/c589862b783b94a8408b40c6dc9bf4a14b2ee391?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `91.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1057      +/-   ##
==========================================
- Coverage    79.6%   79.59%   -0.01%     
==========================================
  Files          42       42              
  Lines        6863     6867       +4     
==========================================
+ Hits         5463     5466       +3     
- Misses       1400     1401       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfcm9iZXJ0YS5weQ==) | `75.89% <ø> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfeGxuZXQucHk=) | `79.01% <ø> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfdHJhbnNmb194bC5weQ==) | `57.53% <ø> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfb3BlbmFpLnB5) | `74.76% <ø> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_xlm.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfeGxtLnB5) | `86.66% <ø> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfYmVydC5weQ==) | `87.98% <ø> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfZ3B0Mi5weQ==) | `75.84% <ø> (ø)` | :arrow_up: |
| [pytorch\_transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX2JlcnQucHk=) | `94.83% <0%> (-0.45%)` | :arrow_down: |
| [pytorch\_transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfdXRpbHMucHk=) | `83.33% <100%> (+0.08%)` | :arrow_up: |
| [pytorch\_transformers/file\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvZmlsZV91dGlscy5weQ==) | `71.22% <100%> (ø)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057?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/pytorch-transformers/pull/1057?src=pr&el=footer). Last update [c589862...6d0aa73](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1057?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).