improve performance of momentum (#30881)

python/paddle/optimizer/momentum.py

@@ -104,7 +104,7 @@ class Momentum(Optimizer):
             raise ValueError("learning_rate is not set")
         if momentum is None:
             raise ValueError("momentum is not set")
-        predicate = lambda regular: isinstance(regular, L2DecayRegularizer)
+        predicate = lambda regular: isinstance(regular, (L2DecayRegularizer, float))
         py_regular = None if predicate(weight_decay) else weight_decay
         super(Momentum, self).__init__(
             learning_rate=learning_rate,
@@ -120,6 +120,9 @@ class Momentum(Optimizer):
         if (isinstance(weight_decay, L2DecayRegularizer)):
             self._regularization_method = "l2_decay"
             self._regularization_coeff = weight_decay._regularization_coeff
+        if (isinstance(weight_decay, float)):
+            self._regularization_method = "l2_decay"
+            self._regularization_coeff = weight_decay
         self._multi_precision = multi_precision
         self._rescale_grad = rescale_grad
         self._master_weights = {}