fix lr scheduler docs (#1446)

Co-authored-by: Nicki Skafte <nugginea@gmail.com>

docs/source/optimizers.rst

@@ -13,23 +13,29 @@ Every optimizer you use can be paired with any `LearningRateScheduler <https://p
 
    # Adam + LR scheduler
    def configure_optimizers(self):
-      return [Adam(...)], [ReduceLROnPlateau()]
+      optimizer = Adam(...)
+      scheduler = ReduceLROnPlateau(optimizer, ...)
+      return [optimizer], [scheduler]
 
    # Two optimziers each with a scheduler
    def configure_optimizers(self):
-      return [Adam(...), SGD(...)], [ReduceLROnPlateau(), LambdaLR()]
+      optimizer1 = Adam(...)
+      optimizer2 = SGD(...)
+      scheduler1 = ReduceLROnPlateau(optimizer1, ...)
+      scheduler2 = LambdaLR(optimizer2, ...)
+      return [optimizer1, optimizer2], [scheduler1, scheduler2]
 
    # Same as above with additional params passed to the first scheduler
    def configure_optimizers(self):
       optimizers = [Adam(...), SGD(...)]
       schedulers = [
          {
-            'scheduler': ReduceLROnPlateau(mode='max', patience=7),
+            'scheduler': ReduceLROnPlateau(optimizers[0], ...),
             'monitor': 'val_recall', # Default: val_loss
             'interval': 'epoch',
             'frequency': 1
          },
-         LambdaLR()
+         LambdaLR(optimizers[1], ...)
       ]
       return optimizers, schedulers