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>