[Wav2Vec2] Fix None loss in docstring for Wav2Vec2ForPretraining
# What does this PR do?

- [ ]  This PR fix None loss in docstring for Wav2Vec2ForPretraining
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19061). All of your documentation changes will be reflected on that endpoint.<|||||>Hey @abdouaziz! Looks like you need to rebase onto main: https://github.com/huggingface/transformers/pull/18960#issuecomment-1248291349

That should fix the file changes!