llama : lookup word in vocab before doing BPE merges (#7193)

* fix: llama-3 ignore_merges

* test: add test for llama-3 bpe ignore_merges

* fix: set ignore_merges only for llama-3

* fix: test-tokenizer-1-bpe --ingore-merges detection

* fix: copy to fix fallthrough

* fix: change ignore_merges to bool

* fix: add ignore merges tests to cmake

* llama : alternative merge ignore logic

---------

Co-authored-by: Haoxiang Fei <feihaoxiang@idea.edu.cn>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>