llama : add OpenELM support (#7359)

* Initial OpenELM support (270M only so far)

* Fill out missing entries in llama_model_type_name

* fixup! Initial OpenELM support (270M only so far)

Fix formatting

* llama : support all OpenELM models

* llama : add variable GQA and variable FFN sizes

Some metadata keys can now also be arrays to support setting
their value per-layer for models like OpenELM.

* llama : minor spacing changes

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

* llama : use std::array for per-layer hparams

* llama : fix save/load state

* llama : do not print hparams for vocab-only models

* llama : handle n_head == 0

* llama : use const ref for print_f and fix division by zero

* llama : fix t5 uses of n_head and n_ff

* llama : minor comment

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Co-authored-by: Francis Couture-Harpin <git@compilade.net>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>