[pip package] feature request: pipeline.generate: add ability to get the state, if it was not provided

Right now if I want to have state after the generation, PIPELINE.generate doesn't allow it if I didn't have it already: default value is None

So I need to do something silly like generate a single token with model.forward and pass state from there or make model wrapper which has forward and remembers the state.

So possible solutions to get the model state from generate:

• Add return_state argument and be like

if return_state: return out_str, state
return out_str 

Major drawback: it breaks clear simple API as now there are two return types, which will make LSPs/linters/etc unhappy and they will complain

• Add argument callback_with_state: bool and if it's true call callback(tmp, state) rather than callback(tmp)

Drawback: it's not always called, but I'm not sure it's that big deal

• Rename function to generate_with_state which returns out_str and state. generate replace with a generate_with_state(...)[0]

This will keep the existing API

• Store the state. Make a field PIPELINE.last_state and store state there.

Wastes memory when is not always desirable.