Fix LR scheduler behaviour with AMP

Author: milesial

src/pytorch_lightning/core/module.py

@@ -1621,6 +1621,10 @@ def lr_scheduler_step(self, scheduler, optimizer_idx, metric):
                 scheduler.step(epoch=self.current_epoch)
 
         """
+        optimizer = self.trainer.optimizers[optimizer_idx]
+        if hasattr(optimizer, '_step_count') and optimizer._step_count <= 0:
+            return
+
         if metric is None:
             scheduler.step()  # type: ignore[call-arg]
         else: