Fix `ReduceLROnPlateau` object has no attribute 'get_last_lr' (#23944)

* Fix 'ReduceLROnPlateau' object has no attribute 'get_last_lr'

* fix style

src/transformers/trainer_pt_utils.py

@@ -851,7 +851,10 @@ def _get_learning_rate(self):
             else:
                 raise
     else:
-        last_lr = self.lr_scheduler.get_last_lr()[0]
+        if isinstance(self.lr_scheduler, torch.optim.lr_scheduler.ReduceLROnPlateau):
+            last_lr = self.optimizer.param_groups[0]["lr"]
+        else:
+            last_lr = self.lr_scheduler.get_last_lr()[0]
         if torch.is_tensor(last_lr):
             last_lr = last_lr.item()
     return last_lr