fix ReduceLROnPlateau import from pytorch-lightning

[GaetanLepage]

What does this PR do?

With the latest pytorch-lightning 2.5.6, ReduceLROnPlateau cannot be imported from lightning.fabric.utilities.types.
Relevant change upstream: Lightning-AI/pytorch-lightning@2.5.5...2.5.6#diff-3c91827ebfece21205d023a4f9b5e1d968ec5b0433b55afdf56810feac32ce30L31

This PR directly imports it from torch.optim.lr_scheduler.
The behavior is unchanged.

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

None

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[speediedan]

Thanks for noticing @GaetanLepage! I'll bump the lightning dev sha on the main branch and then cut a new patch release tomorrow with the update.