optimize momentum to speedup dygraph, a little, test=develop (#30099)

python/paddle/optimizer/momentum.py

@@ -198,10 +198,6 @@ class Momentum(Optimizer):
 
         velocity_acc = self._get_accumulator(self._velocity_acc_str,
                                              param_and_grad[0])
-        find_master = self._multi_precision and param_and_grad[
-            0].dtype == core.VarDesc.VarType.FP16
-        master_weight = (self._master_weights[param_and_grad[0].name]
-                         if find_master else None)
         lr = self._create_param_lr(param_and_grad)
 
         if framework.in_dygraph_mode():
@@ -213,6 +209,11 @@ class Momentum(Optimizer):
                 self._regularization_coeff)
             return None
 
+        find_master = self._multi_precision and param_and_grad[
+            0].dtype == core.VarDesc.VarType.FP16
+        master_weight = (self._master_weights[param_and_grad[0].name]
+                         if find_master else None)
+
         attrs = {
             "mu": self._momentum,
             "use_nesterov": self._use_nesterov,