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>