Make ._update_count and ._update_called public

[faroit]

🚀 Feature

running compute on a metric that hasn't been updated can lead to unforseen consequences. Currently a warning is raised in

https://github.com/Lightning-AI/metrics/blob/ed2249d1e250dbfc4f2e654b455f43f3d29abfb1/src/torchmetrics/metric.py#L510-L516

but in some cases a user want's to check the metric's state first to avoid running into issues.
Torchmetrics should offer the state as a public property to the users and callbacks.

Motivation

Running compute() before update() from lightning's ...epoch_ends callbacks is difficult to implement with multiple dataloaders and multiple metrics: its difficult to keep track of which metrics have been updated already.

Pitch

torchmetrics already has an api to check this, which is used by lightning internally.

https://github.com/Lightning-AI/metrics/blob/7cab17c680a3970a49d8a765878e28263e6dc302/src/torchmetrics/metric.py#L142-L145

This api is indicated as private, so I would propose to make this public to maintain support.

[github-actions]

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

[faroit]

@SkafteNicki thanks for doing this!