DO NOT SIGN ANY SORT OF CONTRACT FOR SOMETHING YOU DO NOT EVEN WANT
If this is a legitimate, well intentioned offer to consider taking you back, then there's absolutely no way that benefits from your signature. A willingness to consider taking you back is just that - a willingness - and it's only practically meaningful if at a future time both you and the employer end up interested in pursuing it.
In the meantime, any insistence on your signature is a serious red flag - there's nothing to be gained by binding yourself to this employer you are presently determined to leave, or them to you in a way that you would be unable to enforce or enjoy anyway. If they want to list conditions that limit the situations in which they would take you back, they are free to do so unilaterally. But put simply, if they are honest about having your mutual best interest in mind, they will be able to understand from a single, polite objection why it is pointless for you to sign anything, and accept your refusal. If they push the issue, it indicates there's something more going on - either a shady purpose, or a serious failure of understanding. In either case, that would be a sign that it is time to disengage even more actively than you had already been planning to.
Best case - they make the offer, you say "thank you, I will keep this in mind" and in a few months you feel better, reach out and they take you back.
Intermediate case - one party ends up interested in renewing the relationship, the other is not, so nothing happens - but that is also for the best, because trying to use a previous agreement to force employment when either party is unwilling will not be a positive outcome
Worst case - they do not have your best interest in mind, and leaving them behind with no remaining ties was the right choice.
(For that matter, unless you are being given something new of useful value in return, you should not sign any sort of agreement, acknowledgement, or anything beyond the letter of resignation comprising your own words when leaving a position. Any terms or restrictions you were previously bound by do not need another signature; anything you were not previously subject to is definitely not an obligation to take on when there is no longer anything to be gained in return for doing so. People may make bogus arguments why you should, but these are empty - they are still obligated to provide your final paycheck without delay)