[metrics] reuse metrics in a progress bar
🚀 Feature
reduce duplicated metrics in general output dict and progress bar
Motivation
it can be confusing for a user to passing the same metric multiple times in single output
Pitch
in such case just specify key/name of metric shown in progress bar... but still it would make it less flexible to show what you want
just having {'loss': None, "my-var": 123}  and none mean that it takes from the general/upper-level dict?
Additional context
https://pytorch-lightning.slack.com/archives/CQXV8BRH9/p1585479078063900