Fix missing head_dim in llama config from gguf model
Author: Isotr0py
Pull request: yes

# What does this PR do?

- This PR fixes missing `head_dim` in llama/mistral config extracted from gguf model.
- Refer to llama.cpp [convert_hf_to_gguf.py#L1527-L1536](https://github.com/ggerganov/llama.cpp/blob/master/convert_hf_to_gguf.py#L1527-L1536)

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Fixes # (issue)


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