fix(tokenizer): handle fast tokenizer properly for bos/eos

[NanoCode012]

Reading through the FastLlama tokenizer docs, they stated:

If you want to change the bos_token or the eos_token, make sure to specify them when initializing the model, or call tokenizer.update_post_processor() to make sure that the post-processing is correctly done (otherwise the values of the first token and final token of an encoded sequence will not be correct). For more details, checkout [post-processors] (https://huggingface.co/docs/tokenizers/api/post-processors) documentation.

Ref: https://huggingface.co/docs/transformers/main/en/model_doc/llama#transformers.LlamaTokenizerFast