SPLIT PR: eos bos tokens

[itazap]

Fix for 2 issues:

1. add_bos_token & add_eos_token flags ignored for PreTrainedTokenizerFast: issue discussed here and here
2. add_special_tokens does not update bos_token or eos_token - ex .add_special_tokens({'bos_token': '<new_bos>'})

TASKS:

• [x] added an update_post_processor function in PreTrainedTokenizerFast based on llamatokenizer, allows reading of bos / eos token flag

**SUPPORTS FAST ONLY slow required updating kwargs to be passed into sp_model , so that bos/eos tokens can be added accordingly..

Reviewer: @ArthurZucker

NOTE: hub token seems to not have access to llama 3, should pass after addressed

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.