grammar : check the full vocab only if necessary (opt) (#4306)

* Check the full vocab for grammar only if necessary

* Fix missing logit restoration step (?)

Does this matter, actually?

* Fix whitespace / formatting

* Adjust comment

* Didn't mean to push test gbnf

* Split sampling into the helper function (?)

And also revert the changes made to the header

* common : fix final newline

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