Fix the return documentation rendering for all model outputs
All PyTorch model outputs are documented from their output types. A problem is that just using the docstrings of the output class doesn't render properly on sphinx (this was also the case before the new model outputs were introduced).

This PR adds a function that converts the args part of the docstrings of the output class to render properly on our doc. You can see the transformation by looking [here](https://huggingface.co/transformers/master/model_doc/bert.html#transformers.BertModel.forward) for the docs before this PR and [here](https://64423-155220641-gh.circle-artifacts.com/0/docs/_build/html/model_doc/bert.html#transformers.BertModel.forward) for after it's merged (it's one example, but it will give the same result for all models). Scroll a bit to get to the return part of the doc.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6022?src=pr&el=h1) Report
> Merging [#6022](https://codecov.io/gh/huggingface/transformers/pull/6022?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3996041d0ae23ce23dfb8a343e6344f2f8d54c16&el=desc) will **increase** coverage by `0.40%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6022/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6022?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6022      +/-   ##
==========================================
+ Coverage   78.29%   78.70%   +0.40%     
==========================================
  Files         146      146              
  Lines       26249    26268      +19     
==========================================
+ Hits        20552    20674     +122     
+ Misses       5697     5594     -103     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6022?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6022/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsLnB5) | `79.12% <ø> (ø)` | |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6022/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.20% <100.00%> (+1.00%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6022/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.98% <0.00%> (-49.38%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6022/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.21% <0.00%> (ø)` | |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6022/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.25% <0.00%> (+1.29%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6022/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `95.18% <0.00%> (+74.91%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6022?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/6022?src=pr&el=footer). Last update [3996041...345372d](https://codecov.io/gh/huggingface/transformers/pull/6022?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
