Fix step idx logging and eval frequency

[ntoxeg]

Step index is incorrectly logged as a metric, it should be passed as the step keyword argument to logger.log, otherwise the logger (W&B, etc.) will just assume 1:1 indexing. This is a problem for anyone using logging frequency different than 1.

Additionally, I’ve changed eval logic to only proc if the step index is divisible by 10, so that eval doesn’t run on every step.