Update the error message for the margin_ranking_loss

Update the error message for the margin_ranking_loss

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

@@ -177,6 +177,16 @@ class MarginRakingLossError(unittest.TestCase):
 
         self.assertRaises(ValueError, test_margin_value_error)
 
+        def test_functional_margin_value_error():
+            x = paddle.static.data(name="x", shape=[10, 10], dtype="float64")
+            y = paddle.static.data(name="y", shape=[10, 10], dtype="float64")
+            label = paddle.static.data(
+                name="label", shape=[10, 10], dtype="float64")
+            result = paddle.nn.functional.margin_ranking_loss(
+                x, y, label, margin=0.1, reduction="reduction_mean")
+
+        self.assertRaises(ValueError, test_functional_margin_value_error)
+
 
 if __name__ == "__main__":
     unittest.main()

python/paddle/nn/functional/loss.py

@@ -338,6 +338,10 @@ def margin_ranking_loss(input,
             loss = paddle.nn.functional.margin_ranking_loss(input, other, label)
             print(loss.numpy()) # [0.75]
     """
+    if reduction not in ['sum', 'mean', 'none']:
+        raise ValueError(
+            "The value of 'reduction' in MarginRankingLoss should be 'sum', 'mean' or 'none', but "
+            "received %s, which is not allowed." % reduction)
     if fluid.framework.in_dygraph_mode():
         out = core.ops.elementwise_sub(other, input)
         out = core.ops.elementwise_mul(out, label)