convert : allow conversion of Mistral HF models (#6144)
* Allow conversion of Mistral HF models
* Homogenize Llama, Mistral, Mixtral under the same entry.
* Fix tokenizer, permute tensors
* Use sentencepiece tokenizer, or fall back to hfft.
* convert-hf : small fix for mypy
* convert-hf : fix duplicated block_count
* convert-hf : add vocab size to metadata
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Co-authored-by: Jared Van Bortel <jared@nomic.ai>
