avoid nan loss in masked lm
Author: sanderland
Pull request: yes

# What does this PR do?

* Fixes a case where the loss returned from a masked LM head can be NaN, when the collator hasn't masked anything. This is mainly a risk at low batch sizes and short texts.
* This likely affects quite a lot of models, and may be the primary cause of infrequent nan warnings when tuning models.

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## Before submitting
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Did you write any new necessary tests?
   - Not yet, but tested on a small example, and confirmed NaN losses in validation and training are gone. 
- [ ] If approved, port to other LM heads

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_16522). All of your documentation changes will be reflected on that endpoint.

@LysandreJik please have a look and tell me if this is something you think should be expanded or ignored. Thanks.

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.