fix(PatchTST): Wrong dropout used for PretainHead
Author: maxstrobel
Pull request: yes

# What does this PR do?

Instead of `config.head_dropout` the wrong `config.dropout` is used. Adapt this for the `PatchTSTMaskPretrainHead` to be consistent with the other heads.
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Comments:
@LysandreJik - how can we get the two missing approvals?

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_31117). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.