Changes in preparation to #8578

[carmocca]

What does this PR do?

This PR includes an assortment of changes which are not relevant by themselves but are supposed to make reviewing #8578 easier.

Part of #8578

Does your PR introduce any breaking changes? If yes, please list them.

Removes FitLoop.current_epoch. The property will have no purpose after #8578. Also having a setter there is dangerous.

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cc @Borda @justusschock @awaelchli @akihironitta @rohitgr7 @carmocca @ananthsub @ninginthecloud

[tchaton]

LGTM !