fix doc for LarsMomentumOptimizer test=develop (#18208)

python/paddle/fluid/optimizer.py

@@ -1070,8 +1070,22 @@ class LarsMomentumOptimizer(Optimizer):
     Examples:
         .. code-block:: python
 
-            optimizer = fluid.optimizer.LarsMomentum(learning_rate=0.2, momentum=0.1, lars_weight_decay=0.001)
-            optimizer.minimize(cost)
+            import paddle.fluid as fluid
+            import numpy as np
+
+            np_inp = np.array([[1.0, 2.0], [3.0, 4.0]], dtype=np.float32)
+            inp = fluid.layers.data(
+                name="inp", shape=[2, 2], append_batch_size=False)
+            out = fluid.layers.fc(inp, size=3)
+            out = fluid.layers.reduce_sum(out)
+            optimizer = fluid.optimizer.LarsMomentumOptimizer(learning_rate=0.001, momentum=0.9)
+            optimizer.minimize(out)
+
+            exe = fluid.Executor(fluid.CPUPlace())
+            exe.run(fluid.default_startup_program())
+            exe.run(
+                feed={"inp": np_inp},
+                fetch_list=[out.name])
     """
     _velocity_acc_str = "velocity"