Support stepping options for lr scheduler

[williamFalcon]

Currently schedulers get called every epoch. Sometimes though, we want them to be called every step.

Proposal 1:
Allow configure_optimizers to return this:

return Adam, {'scheduler': LRScheduler, 'interval': 'batch|epoch'}

@ethanwharris @Borda thoughts? any simpler more general way of doing this? i think this dict can eventually have more options if we need to.

@srush

[Borda]

@SkafteNicki is working on it... Thx!