New `sdialog.tasks` module for LLM-based Input->output tasks

[sergioburdisso]

It would be very useful to have a base class for any Input->output tasks using LLMs.

This will allow to create sub-classes for specific tasks (e.g. entity recognition, keyword extraction, sentiment classification, etc.).

In fact, having a new module sdialog.tasks for this would be useful, then we can have subclasses for different tasks.