change metaclass of Layer from pybind11_builtins.pybind11_type to type (#35538)

* change metaclass of Layer from pybind11_builtins.pybind11_type to type

* fix cast

* add ut

Author: zhiqiu

paddle/fluid/pybind/imperative.cc

@@ -1791,7 +1791,11 @@ void BindImperative(py::module *m_ptr) {
       .def_property_readonly("type", &imperative::VarBase::Type)
       .def_property_readonly("dtype", &imperative::VarBase::DataType);
 
-  py::class_<imperative::Layer, Layer /* <--- trampoline*/> layer(m, "Layer");
+  // NOTE(zhiqiu): set the metaclass of Layer.
+  // See details: https://github.com/pybind/pybind11/pull/679
+  // https://github.com/pybind/pybind11/blob/028812ae7eee307dca5f8f69d467af7b92cc41c8/tests/test_methods_and_attributes.cpp#L284
+  py::class_<imperative::Layer, Layer /* <--- trampoline*/> layer(
+      m, "Layer", py::metaclass((PyObject *)&PyType_Type));  // NOLINT
   layer.def(py::init<>())
       .def("forward",
            [](imperative::Layer &self,

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

@@ -841,6 +841,14 @@ def test_without_guard(self):
             y = fluid.layers.matmul(x, x)
 
 
+class TestMetaclass(unittest.TestCase):
+    def test_metaclass(self):
+        self.assertEqual(type(MyLayer).__name__, 'type')
+        self.assertNotEqual(type(MyLayer).__name__, 'pybind11_type')
+        self.assertEqual(
+            type(paddle.fluid.core.VarBase).__name__, 'pybind11_type')
+
+
 if __name__ == '__main__':
     paddle.enable_static()
     unittest.main()