Remove old timing calls from progressive resizing

Let's update progressive resizing to use the timing abstraction (#146).

Specifically, all calls to:
- `state.epoch`
- `state.step`
- `state.batch_idx`
- `state.max_epochs`
- `state.steps_per_epoch`
 
should be replaced with:
- `state.timer` (and it's methods)
- `state.max_duration`
- `state.get_elapsed_duration()`

As an example, please see #221.