SprintInterface demo() depends on Theano

[JackTemaki]

https://github.com/rwth-i6/returnn/blob/master/returnn/sprint/interface.py#L1025

Minor parts of this still have a Theano dependency, it is unclear if this should be fixed or just everything removed.

[albertz]

It just should be kept, as it is just a demo, i.e. for demonstration purpose. Yes, it would be nicer if it also works, but I think this is not really necessary. So, if anyone cares about it, it could be adapted to TF, but otherwise, it can also just stay as-is. I don't really see a big problem. Maybe you can add a comment.