Allow ckpt module to write stats only

[j-woz]

For IMPROVE results

[jmohdyusof]

Does this mean write stats every timestep regardless of ckpt settings? This seems to make sense to me

[j-woz]

The user can get that with ckpt_save_interval=1 and ckpt_save_weights_file=False

[j-woz]

This works as of 0fb15f7 with no application changes

[jmohdyusof]

What happens if I want to have the stats written and then have a usable best checkpoint? Won't ckpt_save_weights_file=False make the ckpt useless?

[j-woz]

That isn't supported here but I can do that next

[jmohdyusof]

I think the stats output should be on every timestep by default.

[j-woz]

Yes, I can see the use for that

[jmohdyusof]

I think the intent is (was?) to use this for the standard metric output. Maybe I am misremembering, but if I am correct then the stats_every_epoch makes sense