[docs] fix model_doc links in model summary
Possible fix for #5561

@sgugger 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5566?src=pr&el=h1) Report
> Merging [#5566](https://codecov.io/gh/huggingface/transformers/pull/5566?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1d2332861f5225aef17bd7e75abc670a72239081&el=desc) will **increase** coverage by `0.35%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5566/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5566?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5566      +/-   ##
==========================================
+ Coverage   77.20%   77.55%   +0.35%     
==========================================
  Files         141      141              
  Lines       24638    24638              
==========================================
+ Hits        19021    19108      +87     
+ Misses       5617     5530      -87     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5566?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `20.78% <0.00%> (-74.20%)` | :arrow_down: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `61.90% <0.00%> (-33.34%)` | :arrow_down: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `95.55% <0.00%> (-2.23%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `79.72% <0.00%> (-1.38%)` | :arrow_down: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `95.68% <0.00%> (-0.72%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.21% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3V0aWxzLnB5) | `97.11% <0.00%> (+0.28%)` | :arrow_up: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.60% <0.00%> (+1.16%)` | :arrow_up: |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `42.48% <0.00%> (+8.92%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.04% <0.00%> (+49.56%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/huggingface/transformers/pull/5566/diff?src=pr&el=tree-more) | |

------

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<|||||>No, this won't work across versions (if you switch to master docs for instance). The problem is in the first slash, I think removing it should be enough to make all link works.<|||||>Double-checked locally, this is the right fix: `/model_doc/gpt` -> `model_doc/gpt`
 <|||||>should I make these changes ? also model_doc/gpt or model_doc/gpt.html ?<|||||>No need for the .html.
I can force-push on your branch or let you do the changes, your call!<|||||>How much time does it take to these changes be reflected in the website?

Also, can I close my issue after this fix?

@sgugger 

Thanks! :)<|||||>The thing is that the bug will probably stay in the stable version of the doc forever (unless we manage to cherry-pick it somehow @LysandreJik ?). It is reflected in the [master version](https://huggingface.co/transformers/master/model_summary.html) already.

You can close your issue whenever you like :-)