Update the new model template
The model template files were old and did not use any of the recent improvements (output attentions, output hidden states, docstrings refactored...)

This PR fixes them and adds a few more instructions.

Tagging @LysandreJik, @patrickvonplaten, @thomwolf and @julien-c : We have to make sure that whenever we make a substantial change that applies to all model, this template is also updated.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6019?src=pr&el=h1) Report
> Merging [#6019](https://codecov.io/gh/huggingface/transformers/pull/6019?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/778e635fc900e5afc90e5f8f4ca74b5d5fc2976a&el=desc) will **increase** coverage by `1.37%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6019/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6019?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6019      +/-   ##
==========================================
+ Coverage   77.31%   78.68%   +1.37%     
==========================================
  Files         146      146              
  Lines       26249    26249              
==========================================
+ Hits        20295    20655     +360     
+ Misses       5954     5594     -360     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6019?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6019/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.98% <0.00%> (-49.38%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6019/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.90% <0.00%> (-33.90%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6019/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.53% <0.00%> (+69.51%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6019/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6019?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/6019?src=pr&el=footer). Last update [778e635...b79103c](https://codecov.io/gh/huggingface/transformers/pull/6019?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
