Make attention_mask axes dynamic when exporting onnx
# What does this PR do?

Make `attention_mask` axes dynamic when exporting onnx


## Who can review?
[@fatcat-z](https://github.com/fatcat-z)

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17711). All of your documentation changes will be reflected on that endpoint.<|||||>cc @michaelbenayoun @lewtun <|||||>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.<|||||>Hey @unbuilt just checking if you were able to test that the script runs correctly with your change and the default settings? If yes, this looks good to merge IMO :)<|||||>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.