Fix param base trainable set failed (#28756)

* fix param base trainable set failed

* add unittest

* fix typo

* polish comment

python/paddle/fluid/framework.py

@@ -2858,6 +2858,12 @@ class Block(object):
             param = ParamBase(*args, **kwargs)
         else:
             param = Parameter(global_block, *args, **kwargs)
+            # NOTE: Why only set stop_gradient=False in static mode
+            # Because in dygraph mode, the `stop_gradient` and `trainable`
+            # are related, and `trainable` default vallue is `True` or
+            # it is specified by users, there is no need to set
+            # `stop_gradient` for ParamBase here.
+            param.stop_gradient = False
         if 'initializer' in kwargs:
 
             def _is_inited_by(block, var):
@@ -2884,7 +2890,6 @@ class Block(object):
                 pass
             else:
                 initializer(param, self)
-        param.stop_gradient = False
         return param
 
     def append_op(self, *args, **kwargs):

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

@@ -3683,5 +3683,24 @@ class TestMetricsDetectionMap(unittest.TestCase):
         print(str(program))
 
 
+class ExampleNet(paddle.nn.Layer):
+    def __init__(self):
+        super(ExampleNet, self).__init__()
+        self.weight = self.create_parameter(
+            shape=[1, 1], attr=paddle.ParamAttr(trainable=False))
+
+    def forward(self):
+        # only for test parameter trainable attr
+        pass
+
+
+class TestLayerParameterTrainableSet(unittest.TestCase):
+    def test_layer_parameter_set(self):
+        with fluid.dygraph.guard():
+            net = ExampleNet()
+            self.assertFalse(net.weight.trainable)
+
+
 if __name__ == '__main__':
+    paddle.enable_static()
     unittest.main()