[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

[SkafteNicki]

I agree that we need to keep dictionaries for backward compatibility, I just think that if we offer a solution to users that takes care of the problem (Result objects) and it is not that hard to convert from dictionaries, then it is not really a problem :]

[SkafteNicki]

@Borda I think we can close this, since users just need to use the Result objects to remove such duplications