Super tiny fix unused `masked_bias`
Author: fzyzcjy
Pull request: yes

# What does this PR do?

Hi thanks for the library! When learning (thus reading source code) GPT2, it seems that the `masked_bias` is not used. I have searched this keyword in the lib and does not find usages as well. Thus I try to make this PR and see whether the removal still makes CI happy.

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Comments:
@fzyzcjy Running `make fix-copies` and pushing the changes should resolve failing the quality checks

Oops I forgot this... Will do it later when working with https://github.com/huggingface/transformers/issues/30647

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.

@fzyzcjy Any update on this PR? 

Sorry I do not have huggingface transformers environment locally and may not worth it to create one for this single-line PR...

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.