Variable inner loops for different optimizers

[lubbersnick]

If we pull out the step loop from the epoch loop as a variable function, this will make the library compatible with closure-based optimizers or other optimizers (such as SAM).

To do this, we should add three versions

• (default) regular steps
• closure-based steps
• two-step (like SAM)

Which version should be controlled by the Controller. We can create an enum or use a string.