Fix checkpoint values when saving and resetting the tuner state

[carmocca]

What does this PR do?

#8578 requires using the checkpoint connector to manage the trainer state before and after tuning.

Part of #8578

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

None

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

[tchaton]

LGTM !