substitute auto slot filling with customizable action

related to https://github.com/RasaHQ/rasa/issues/6090

Instead of autofill slots in domain.py, create a default action that is unfeaturized and always executed after user utterance unless there is an active_form and by default fill the slots with the same entities, if action_fill_slots is present in the domain, it’ll run it which will allow users to create their own slot-filling logic (basically a cut version of FormAction’s extract->validate). Effectively, this customization will serve as continuation of nlu pipeline but with access to the tracker

[nbeuchat]

I very much like this idea!

I see a few use cases from our side:

• For our apartment search bot, it can help us set (or edit) user preferences outside of our standard preferences form. The reason we cannot use automatic slot filling is that users may use the same entities (ex: price, unit type, etc.) to select an apartment (ex: I want to schedule a viewing for the two-bedroom apartment you showed me) or to set the preferences (ex: Can you show me 2 bedroom apartments?). In the first case, we do not want to auto-fill the slots while in the second, we want to. I know you can also predict an action to change preferences in the second case and use the extracted entities but the auto slot filling would not rely on this prediction.
• The other use-case as @Ghostvv mentioned would be to add context to our bot like the user city to correctly disambiguate neighborhoods that the user may mention (in this case, we would also need to do the same disambiguation in the preferences form but that’s ok as it’s only two places instead of everywhere where the bot needs this info)

[kengelbrecht]

Awesome! Hope 3.0 will be released soon 😃