Re-examine the whole state story for RNNs

[MischaPanch]

The state: Any | None = None crept into all forwards of children of NetBase. Properly accommodating it forced me to make NetBase generic, just to allow to do class Recurrent(NetBase[RecurrentStateBatch]).

The state is unused almost everywhere and it makes every signature more complicated. On top of that, RNN things are currently broken anyway. Maybe there's a way to make our lives simpler without completely throwing over board the possibility to repair RNNs down the line?

Also pinging @opcode81 @Trinkle23897 , do you have an opinion?