[WIP][Feature][Bugfix] WandB setup/finish simplified + callback.

[sami-bg]

Description

Extracts all wandb lifecycle logic (init, config sync, offline data dump, wandb.finish()) from Manager into a new WandbCallback that uses Lightning's setup()/teardown() hooks, so wandb.finish() is called automatically after each trainer stage. This fixes run leaks in multirun sweeps (fit then test) without requiring manual cleanup

Moves WandbCheckpoint out of checkpoint_sklearn.py into wandb_lifecycle.py alongside WandbCallback, and fixes its docstring which was copy-pasted from the sklearn checkpoint

Adds unit tests covering config flattening, offline file I/O, SLURM requeue directory discovery, setup/teardown state management, on_exception crash recovery, and checkpoint run-ID continuity

Checklist

• I have read the Contributing document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR to the RELEASES.rst file.

[RandallBalestriero]

Nice, thanks @sami-bg ! did you try running with DDP and preemption/requeue to make sure everything is safely dumped/loaded re-init every time?

[sami-bg]

Not yet! Sorry forgot to add [WIP]. Will test this soon

[RandallBalestriero]

Hi @sami-bg !

I did an update on wandb callbacks in the last few merge, could you please double check and if spt now has all the functionalities you were implementing, please close this PR, thank you!