Allow format_checkpoint_name() to use callback metrics

[minhduc0711]

🚀 Feature

Let users use checkpoint_on and val_checkpoint_on in the checkpoint's filepath when initializing a ModelCheckpoint like this:

ckpt_callback = ModelCheckpoint(filepath="ckpts/{epoch}-{val_checkpoint_on}")

Motivation

Right now, the formatting function only allows selecting from metrics that were logged.

I would like to use metrics from trainer.callback_metrics as well. The reason I can't use logged metrics is that my scalar name contains a forward slash to utilize Tensorboard's grouping, and it would mess up the formatting function.

Pitch

Like what I said at the start.

Alternatives

Or perhaps we can adjust format_checkpoint_name() to handle forward slashes in metrics' names, but I guess that would be a little bit more tricky.

[github-actions]

Hi! thanks for your contribution!, great first issue!