llama : fix n_vocab init for 'no_vocab' case (#9511)

* llama: fixed n_vocab for `no_vocab` models

* llama: updated error output for `llama_decode_internal` and `llama_encode_internal`

* llama: log warning if there's no vocab_size in metadata

* llama: correct vocab size for logging

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

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