Please don't make us specify the number of outputs for R2Score

[profPlum]

🚀 Feature

I'm asking that for R2Score num_outputs argument not be required. Just like it isn't required for ExplainedVariance.

Motivation

There are situations where (e.g. CNN image to image, or generic LightningModule parent class) where I won't know the number of outputs in advance which makes the class impossible to use.

Pitch

It isn't really necessary to ask for it b/c ExplainedVariance doesn't.
Just do whatever ExplainedVariance does to avoid requiring the number of outputs in advance.

[github-actions]

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