[Torchscript] Fix docs
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Fixes #6714

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6740?src=pr&el=h1) Report
> Merging [#6740](https://codecov.io/gh/huggingface/transformers/pull/6740?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/64c7c2bc158364ff5c53dce2f19698078b2f9d78?el=desc) will **decrease** coverage by `0.29%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6740/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6740?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6740      +/-   ##
==========================================
- Coverage   80.00%   79.70%   -0.30%     
==========================================
  Files         156      156              
  Lines       28426    28426              
==========================================
- Hits        22741    22656      -85     
- Misses       5685     5770      +85     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6740?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6740/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `66.66% <0.00%> (-32.50%)` | :arrow_down: |
| [src/transformers/tokenization\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/6740/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxuZXQucHk=) | `66.66% <0.00%> (-23.43%)` | :arrow_down: |
| [src/transformers/tokenization\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/6740/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcmVmb3JtZXIucHk=) | `81.66% <0.00%> (-13.34%)` | :arrow_down: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6740/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `71.21% <0.00%> (-12.88%)` | :arrow_down: |
| [src/transformers/tokenization\_dpr.py](https://codecov.io/gh/huggingface/transformers/pull/6740/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZHByLnB5) | `57.65% <0.00%> (+4.50%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6740?src=pr&el=footer). Last update [64c7c2b...02d9292](https://codecov.io/gh/huggingface/transformers/pull/6740?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
