server : add llama2 chat template (#5425)
* server: add mistral chat template
* server: fix typo
* server: rename template mistral to llama2
* server: format_llama2: remove BOS
* server: validate "--chat-template" argument
* server: clean up using_chatml variable
Co-authored-by: Jared Van Bortel <cebtenzzre@gmail.com>
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Co-authored-by: Jared Van Bortel <cebtenzzre@gmail.com>
