Prohibit GPT-2 from generating some words on a condition


I have added to the GPT-2 vocabulary two special tokens ([ss], [se]) and use it to generate sequences.
I want to prohibit GPT-2 from generating some words after having generated [ss] until it generates [se] . For example in the following sequence

some words of the sequence [ss] here I want some words not to be generated [se] other words of the sequence.

in other words, I want between [ss] … [se] to exclude some words from the generation. As @deathcrush answered here it is possible to prohibit GPT-2 from generating some vocabulary ids in general, but is it possible to do it on condition? (only between those tokens [ss] and [se])

Could you check out this function for contained prefix generation: Models — transformers 4.4.2 documentation

Thank you @patrickvonplaten for your fast reply.

The link you provided is for the models page. Could you point me to the function you are talking about?

@patrickvonplaten I saw that generate function has a prefix_allowed_tokens_fn. Were you referring to this?

Yes, that is what he meant.

This function is used to constrain beam search. I want to generate text with sampling and specifically with top_p. Is there a way to utilise this function?

@patrickvonplaten in the documentation of generate function the description of prefix_allowed_tokens_fn argument says

If provided, this function constraints the beam search to allowed tokens only at each step.

Can this function be applied with sampling as well?