Show a warning for missing attention masks when pad_token_id is not None
# What does this PR do?

Fixes #16136

Shows a one-time warning message when the pad_token_id is not None and no attention masks are given. 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? #16136 
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@gante @ydshieh
@ydshieh @gante

I've added the warning message as per #21916 and also a short test. Please let me know if you have any suggestions. If it looks good, I can copy/paste the warning to more models as well. Thanks!
<|||||>@hackyon LGTM 👍 

But before you spend time propagating the change to the other models, let's also get a green light from a core maintainer (cc @sgugger )<|||||>Thanks for the review. 

Would it be better to check for the presence of the pad_token_id inside input_ids first before throwing the error, as per
https://github.com/huggingface/transformers/pull/17444/files? If so, I can make the change to reflect that here.

<|||||>@sgugger 

Thanks for the input. I changed my pull request up to be more like #17444. Let me know what you think. Thanks!
<|||||>Thanks, I've updated the code accordingly.<|||||>@gante could you have a second look here?<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>@hackyon Thank you for the contribution! Would you like to add it to the remaining models? 🤗 <|||||>Sure, I'll look into it 👍