[Dy2stat] support usage: to_static(model) (#27040)

* support to_static(model)

* add warning and unittest

python/paddle/fluid/dygraph/jit.py

@@ -212,6 +212,15 @@ def declarative(function=None, input_spec=None):
 
     # for usage: `declarative(foo, ...)`
     if function is not None:
+        if isinstance(function, Layer):
+            if isinstance(function.forward, StaticLayer):
+                class_name = function.__class__.__name__
+                warnings.warn(
+                    "`{}.forward` has already been decorated somewhere. It will be redecorated to replace previous one.".
+                    format(class_name))
+            function.forward = decorated(function.forward)
+            return function
+        else:
             return decorated(function)
 
     # for usage: `@declarative`

python/paddle/fluid/tests/unittests/dygraph_to_static/test_declarative.py

@@ -332,5 +332,31 @@ class TestDeclarativeAPI(unittest.TestCase):
             func(np.ones(5).astype("int32"))
 
 
+class TestDecorateModelDirectly(unittest.TestCase):
+    def setUp(self):
+        paddle.disable_static()
+        program_trans.enable(True)
+        self.x = to_variable(np.ones([4, 10]).astype('float32'))
+
+    def test_fake_input(self):
+        net = SimpleNet()
+        net = declarative(net)
+        y = net(self.x)
+        self.assertTrue(len(net.forward.program_cache) == 1)
+
+    def test_input_spec(self):
+        net = SimpleNet()
+        net = declarative(net, input_spec=[InputSpec([None, 8, 10])])
+        self.assertTrue(len(net.forward.inputs) == 1)
+        self.assertTrue(len(net.forward.program_cache) == 1)
+        input_shape = net.forward.inputs[0].shape
+        self.assertListEqual(list(input_shape), [-1, 8, 10])
+
+        # redecorate
+        net = declarative(net, input_spec=[InputSpec([None, 16, 10])])
+        input_shape = net.forward.inputs[0].shape
+        self.assertListEqual(list(input_shape), [-1, 16, 10])
+
+
 if __name__ == '__main__':
     unittest.main()