Add Slow CI reminder bot
Author: ydshieh
Pull request: yes

# What does this PR do?

@amyeroberts and @gante both mentioned that it would be helpful if we have such a reminder (so we won't forget to trigger whenever necessary)

p.s. 

- I am using `comment_bot_token` whose owner is [HuggingFaceDocBuilderDev](https://github.com/HuggingFaceDocBuilderDev), which might seems a bit strange. But I think it's fine (and not to create a new member with a new token again)


<img width="627" alt="Screenshot 2024-09-16 115318" src="https://github.com/user-attachments/assets/d8a5d4c0-a96e-4577-ad8e-a0d1fbe988a8">


Comments:
> Thank you for starting this PR! 🤗
> 
> I have a few suggestions to improve the workflow from both ends (contributor and maintainer):
> 
>     1. State in the text that this is a reminder for the maintainer, not for the contributor 🤗
> 
>     2. Add a link to our notion page, so that we can quickly confirm the commands to run the slow CI
> 
>     3. Not sure if possible/easy, but it would be cool to skip this command if there are no relevant slow tests. At the very least, if no `.py` files were changed

Great suggestions, thank you!


+1 to @gante points 2 & 3

For 1, I think we can put some of this on the contributors to enable (even if they're not responsible for ensuring they've run) to save maintainers some time :) 

We could add a message along the lines of: 
```
Before merging slow tests should be run. To enable this:
* Add the `run-slow` label to the PR
* When your PR is ready for merge and all reviewers' comments have been addressed, push an empty commit with the command `[run-slow]` followed by a comma separated list of all the models to be tested i.e. `[run_slow] model_to_test_1, model_to_test_2`
* A Hugging Face employee will then approve the workflow to start the tests 
``` 



Hey! Comments addressed. I change a bit of wording (cc @amyeroberts). WDYT? It looks like below

------------------------------------------------------------------------------------


Before merging this pull request, slow tests CI should be triggered. To enable this:
- Add the `run-slow` label to the PR
- When your PR is ready for merge and all reviewers' comments have been addressed, push an empty commit with the command `[run-slow]` followed by a comma separated list of all the models to be tested, i.e. `[run_slow] model_to_test_1, model_to_test_2`
- A `transformers` maintainer will then approve the workflow to start the tests

(For maintainers) The documentation for slow tests CI on PRs is [here](https://www.notion.so/huggingface2/CI-for-pull-requests-8335bd9217d24d1e8ed1d3a016f39804).


I think @ArthurZucker's approval would count more than mine on this 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_33506). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Any further comment @ArthurZucker ?

> Yeah a lot better thanks! Can we have @glegendre01 's approval for the security of using outsourced actions?

That action is used in [huggingface/doc-builder](https://github.com/huggingface/doc-builder/blob/1b42854493622c0c8c31579a12078d6c71f5c9d3/.github/workflows/upload_pr_documentation.yml#L122)

```
      - name: Add doc comment if not present
        uses: thollander/actions-comment-pull-request@v2
        if: steps.find_comment.outputs.comment-id == ''

        with:
          message: 'The docs for this PR live [here](${{ steps.hfhub-context.outputs.hub_docs_url }}). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.'
          pr_number: ${{ steps.github-context.outputs.pr_number }}
          GITHUB_TOKEN: ${{ secrets.comment_bot_token }}
```

oh, i forget that one here now, but it's also used already (I checked it before)

https://github.com/huggingface/doc-builder/blob/1b42854493622c0c8c31579a12078d6c71f5c9d3/.github/workflows/upload_pr_documentation.yml#L115

(I won't keep silence when I actually a new action - you can trust me!)

I will wait and merge on Monday instead. Don't want to confuse the contributors during the weekend.

Thanks for review!