LRScheduler.get_lr should not update lr in LinearWarmup (#31843)

python/paddle/fluid/tests/unittests/test_lr_scheduler.py

@@ -537,6 +537,18 @@ class TestLRScheduler(unittest.TestCase):
                 self._test_dygraph(python_func, paddle_api, kwarg, place)
                 paddle.enable_static()
 
+    def test_linear_warmp(self):
+        natural_lr = paddle.optimizer.lr.NaturalExpDecay(
+            learning_rate=0.5, gamma=0.1)
+        natural_lr_warmup = paddle.optimizer.lr.LinearWarmup(
+            learning_rate=natural_lr, warmup_steps=10, start_lr=0.0, end_lr=0.1)
+        for idx in range(30):
+            if idx >= 10:
+                self.assertEqual(natural_lr_warmup.get_lr(),
+                                 natural_lr.get_lr())
+                natural_lr.step()
+            natural_lr_warmup.step()
+
 
 if __name__ == '__main__':
     unittest.main()

python/paddle/optimizer/lr.py

@@ -786,9 +786,8 @@ class LinearWarmup(LRScheduler):
                 self.last_epoch) / float(self.warmup_steps) + self.start_lr
         else:
             if isinstance(self.learning_rate, LRScheduler):
-                lr_value = self.learning_rate()
-                self.learning_rate.step()
-                return lr_value
+                self.learning_rate.step(self.last_epoch - self.warmup_steps)
+                return self.learning_rate()
 
             return self.learning_rate