examples : restore the functionality to import llama2.c models (#2685)

* Fix import of llama2.c models that don't share weights between embedding layers

* llama2c: reinstate ggmlv3 conversion output + update readme w/ gguf conv

* llama2.c: comment out legacy "load from ggml model" logic

* llama2.c: convert special-cased "<0xXX>" single byte tokens from tokenizer.bin