Torturer tries to make you gain a curse (to your discard), then move it from discard to hand. If you use trader to gain a silver instead (to your discard), Torturer "loses track" of the curse and moves nothing to your hand. So you end up with the silver in your discard pile.
That's not correct. It's not quite that complicated. The "lose track" rule is very rarely needed, and it's not needed here.
First of all, Torturer says
"he gains a Curse card, putting it in his hand," which means the Curse goes directly to your hand. This is actually from the first rule book:
"Gain -- when a player gains a card, he takes the gained card (usually from the Supply) and puts it onto his Discard pile (unless the card says to put it elsewhere)." But if you use Trader it doesn't even matter where the Curse would have gone, because the gain never happens. Instead, you get a new gain, which is "you gain a Silver," with no qualifiers. As per the standard rules, this Silver comes from Supply and goes to you discard. This is also explained in the FAQ for Trader.
If hypothecally Torturer had instead said
"he gains a Curse card, and then puts it in his hand," it would be two instructions. So the Curse would go to your discard. In this case the second instruction would indeed try to execute after you've cancelled the first instruction and gained a Silver instead. But even then there's no "lose track" rule needed since the card doesn't even exist. Torturer didn't gain you a Curse, so the second instruction fails. It would be like if the Curse pile was empty; the first instruction fails, so the second can't be resolved.