!3863 Modify logic

Merge pull request !3863 from lijiaqi/bug

mindspore/nn/optim/sgd.py

@@ -134,10 +134,6 @@ class SGD(Optimizer):
         if isinstance(momentum, float) and momentum < 0.0:
             raise ValueError("momentum should be at least 0.0, but got momentum {}".format(momentum))
 
-        if nesterov and (momentum <= 0.0 or dampening != 0.0):
-            raise ValueError("If use nesterov, momentum must be positive and dampening must equal to 0.0,"
-                             "but got momentum {}, dampening {}".format(momentum, dampening))
-
         if isinstance(dampening, int):
             dampening = float(dampening)
         if not isinstance(dampening, float):
@@ -151,6 +147,10 @@ class SGD(Optimizer):
             weight_decay = float(weight_decay)
 
         validator.check_value_type("nesterov", nesterov, [bool], self.cls_name)
+
+        if nesterov and (momentum <= 0.0 or dampening != 0.0):
+            raise ValueError("If use nesterov, momentum must be positive and dampening must equal to 0.0,"
+                             "but got momentum {}, dampening {}".format(momentum, dampening))
         self.nesterov = nesterov
 
         self.opt = P.SGD(dampening, weight_decay, nesterov)