Fix multi tensor momentum regular bug (#38344)

* fix merged_momentum regular bug

* fix bug

python/paddle/optimizer/momentum.py

@@ -192,7 +192,7 @@ class Momentum(Optimizer):
 
     def _update_regularization(self, weight_decay):
         reg_method = ""
-        reg_coeff = 0
+        reg_coeff = 0.0
 
         if (isinstance(weight_decay, L2DecayRegularizer)):
             reg_method = "l2_decay"
@@ -306,7 +306,7 @@ class Momentum(Optimizer):
             # the param's regularization has been done before, we avoid do l2decay in momentum.
             elif param.regularizer is not None:
                 regularization_method = ""
-                regularization_coeff = 0
+                regularization_coeff = 0.0
 
         find_master = self._multi_precision and param_and_grad[
             0].dtype == core.VarDesc.VarType.FP16
@@ -380,7 +380,7 @@ class Momentum(Optimizer):
                 if isinstance(param.regularizer, L2DecayRegularizer):
                     regularization_method = "l2_decay"
                     regularization_coeff = param.regularizer._regularization_coeff
-                else:
+                elif param.regularizer is not None:
                     regularization_method = ""
                     regularization_coeff = 0.0
             if param.dtype == paddle.float32: