Add a GenerativeTask base class

[mattdangerw]

With #804, we are moving generative compilation into the task model.

We should add a base class for all generative tasks models that can pull out some of the common compilation code for tasks with a generate method.

[jbischof]

Is the idea Task -> GenerativeTask -> GPTCausalLM?

Of course, if a Task ever needs two traits we might be stuck in mixin territory. Let's hope 🤞

[mattdangerw]

Is the idea Task -> GenerativeTask -> GPTCausalLM?

Yeah probably! I don't think it makes sense to put this stuff on the task base class. So as you say, it's that or mixins.

[mattdangerw]

This is fixed!