fix bug for MultiplicativeDecay (#41850)

* fix bug for MultiplicativeDecay

* remove changes to test_lr_scheduler.py

python/paddle/optimizer/lr.py

@@ -1587,7 +1587,7 @@ class MultiplicativeDecay(LRScheduler):
                                                   verbose)
 
     def get_lr(self):
-        if self.last_epoch > 0:
-            return self.last_lr * self.lr_lambda(self.last_epoch)
-        else:
-            return self.base_lr
+        cur_lr = self.base_lr
+        for epoch in range(1, self.last_epoch + 1):
+            cur_lr = cur_lr * self.lr_lambda(epoch)
+        return cur_lr