Add head_mask for llama
Author: fxmeng
Pull request: yes

# What does this PR do?
Support inputting a head_mask to LLaMA's forward like other models.
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Fixes # (issue)


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23109). All of your documentation changes will be reflected on that endpoint.

cc @ArthurZucker and @younesbelkada