It doesn’t need to be a torpedo. You can put a pretty big device inside an unmanned submersible. The transporting sub doesn’t need to do more than drop it off and scoot.
I know that but a torpedo can deliver a device very easily too. Can be modified to do a bunch of things and is conveniently carried on an SSN. Can be used in a multi-purpose role. BTW Russia is going smaller and more accurate so far as its nuclear weapons are concerned.