"Live" model modification

[lucasb-eyer]

We initially discussed this, I just put it up so we don't forget about it. We wanted the model to notice when it changes, e.g. a module is added to a container, and invalidate all state optimizers stored in it, so that they are forced to recompile. We shortly thought about a dirty flag or implementing and somehow using __hash__.

I have not further thought about it, I even almost forgot it, hence me filing this issue.

[ikostrikov]

Yes. It doesn't look difficult to implement, so I suggest to keep it as it is for a while.