Much as I would be offended by and object to a pardon, it wouldn't prevent prosecution of the perpetrators, as every country on the planet has the legal authority to try cases of torture, regardless where the perpetrator came from, and regardless what the perpetrator's country says about it.

Otherwise, the first step necessary toward prosecution would be investigation, which is essentially that which is being proposed as to a “Commission”. But that the US might promise not to prosecute the perpetrators is not a promise for the rest of the world.

First, get the facts.