Implement __getitem__ as "metric arithmetic"

[victorjoos]

🚀 Feature

Allow a user to define a new metric that takes an item out of an other metric.

Basically :

iou = IoU(num_classes=2, reduction="none")
fg_iou = iou[0]
bg_iou = iou[1]

Motivation

There are multiple metrics (like IoU and confusion matrix) that would benefit from the use of such a feature, and it is close to the mechanism of metric arithmetic.

Pitch

This would only need to define

class Metric:
    ...
    def __getitem__(self, idx):
        return CompositionalMetric(lambda x: x[idx], self, None)

Alternatives

The straightforward alternative is to use CompositionalMetric directly.

[github-actions]

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

[SkafteNicki]

I think this great addition. @justusschock opinion?