Fix LRScheduler import for PyTorch 2.0 (#15940)

* Fix LRScheduler import for PyTorch 2.0
* Add comment for posterity

src/pytorch_lightning/utilities/types.py

@@ -27,6 +27,13 @@
 from torchmetrics import Metric
 from typing_extensions import Protocol, runtime_checkable
 
+try:
+    from torch.optim.lr_scheduler import LRScheduler as TorchLRScheduler
+except ImportError:
+    # For torch <= 1.13.x
+    # TODO: Remove once minimum torch version is 1.14 (or 2.0)
+    from torch.optim.lr_scheduler import _LRScheduler as TorchLRScheduler
+
 from lightning_lite.utilities.types import _LRScheduler, ProcessGroup, ReduceLROnPlateau
 
 _NUMBER = Union[int, float]
@@ -111,9 +118,9 @@ def no_sync(self) -> Generator:
 
 
 # todo: improve LRSchedulerType naming/typing
-LRSchedulerTypeTuple = (torch.optim.lr_scheduler._LRScheduler, torch.optim.lr_scheduler.ReduceLROnPlateau)
-LRSchedulerTypeUnion = Union[torch.optim.lr_scheduler._LRScheduler, torch.optim.lr_scheduler.ReduceLROnPlateau]
-LRSchedulerType = Union[Type[torch.optim.lr_scheduler._LRScheduler], Type[torch.optim.lr_scheduler.ReduceLROnPlateau]]
+LRSchedulerTypeTuple = (TorchLRScheduler, torch.optim.lr_scheduler.ReduceLROnPlateau)
+LRSchedulerTypeUnion = Union[TorchLRScheduler, torch.optim.lr_scheduler.ReduceLROnPlateau]
+LRSchedulerType = Union[Type[TorchLRScheduler], Type[torch.optim.lr_scheduler.ReduceLROnPlateau]]
 LRSchedulerPLType = Union[_LRScheduler, ReduceLROnPlateau]