Normalize only if needed
Author: mjamroz
Pull request: yes

# Normalize tensor only if `std/mean` defined.

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Fixes # (issue)


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Comments:
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.

Could you take a look at this @rafaelpadilla ?

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_26049). All of your documentation changes will be reflected on that endpoint.