update api

test=develop

paddle/fluid/imperative/layer.h

@@ -153,7 +153,10 @@ class Layer {
     return vars;
   }
 
-  virtual void Backward() { LOG(ERROR) << "To support customize"; }
+  virtual std::vector<VarBase> Backward(const std::vector<VarBase>& inputs) {
+    std::vector<VarBase> vars;
+    return vars;
+  }
 };
 
 }  // namespace imperative

paddle/fluid/pybind/imperative.h

@@ -22,7 +22,7 @@ limitations under the License. */
 namespace paddle {
 namespace pybind {
 
-class PyLayer : public imperative::Layer {
+class Layer : public imperative::Layer {
  public:
   using imperative::Layer::Layer;  // Inherit constructors
 
@@ -32,8 +32,10 @@ class PyLayer : public imperative::Layer {
                       inputs);  // NOLINT
   }
 
-  void Backward() override {
-    PYBIND11_OVERLOAD(void, Layer, Backward, );  // NOLINT
+  std::vector<imperative::VarBase> Backward(
+      const std::vector<imperative::VarBase>& inputs) override {
+    PYBIND11_OVERLOAD(std::vector<imperative::VarBase>, Layer, Backward,
+                      inputs);  // NOLINT
   }
 };
 

paddle/fluid/pybind/pybind.cc

@@ -170,14 +170,17 @@ PYBIND11_MODULE(core, m) {
           },
           py::return_value_policy::reference);
 
-  py::class_<imperative::Layer, PyLayer /* <--- trampoline*/> layer(m, "Layer");
+  py::class_<imperative::Layer, Layer /* <--- trampoline*/> layer(m, "Layer");
   layer.def(py::init<>())
       .def("forward",
            [](imperative::Layer &self,
               const std::vector<imperative::VarBase> &inputs) {
              return self.Forward(inputs);
            })
-      .def("backward", &imperative::Layer::Backward);
+      .def("backward", [](imperative::Layer &self,
+                          const std::vector<imperative::VarBase> &inputs) {
+        return self.Backward(inputs);
+      });
   BindTracer(&m);
 
   py::class_<Tensor>(m, "Tensor", py::buffer_protocol())

python/paddle/fluid/imperative/layers.py

@@ -20,10 +20,12 @@ from paddle.fluid import core
 from paddle.fluid import framework
 from paddle.fluid.imperative import base
 
-__all__ = ['PyLayer']
+__all__ = ['Layer']
 
 
-class PyLayer(core.Layer):
+class Layer(core.Layer):
+    """Layers composed of operators."""
+
     def __init__(self, dtype=core.VarDesc.VarType.FP32, name=None):
         self._once_built = False
         self._dtype = dtype
@@ -37,8 +39,23 @@ class PyLayer(core.Layer):
             self._once_built = True
 
         outputs = self.forward(*inputs)
-
         return outputs
 
     def forward(self, *inputs):
         raise NotImplementedError
+
+    def backward(self, *inputs):
+        raise ValueError("Layer shouldn't implement backward")
+
+
+class PyLayer(core.Layer):
+    """Layers composed of user-defined python codes."""
+
+    def __call__(self, *inputs):
+        pass
+
+    def forward(self, *inputs):
+        raise NotImplementedError
+
+    def backward(self, *inputs):
+        raise NotImplementedError

python/paddle/fluid/imperative/nn.py

@@ -30,7 +30,7 @@ __all__ = [
 ]
 
 
-class Conv2D(layers.PyLayer):
+class Conv2D(layers.Layer):
     def __init__(self,
                  num_channels,
                  num_filters,
@@ -143,7 +143,7 @@ class Conv2D(layers.PyLayer):
         return self._helper.append_activation(pre_act)
 
 
-class Pool2D(layers.PyLayer):
+class Pool2D(layers.Layer):
     def __init__(self,
                  pool_size=-1,
                  pool_type="max",
@@ -205,7 +205,7 @@ class Pool2D(layers.PyLayer):
         return pool_out
 
 
-class FC(layers.PyLayer):
+class FC(layers.Layer):
     def __init__(self,
                  size,
                  param_attr=None,

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

@@ -22,7 +22,7 @@ from paddle.fluid.imperative.nn import FC
 from test_imperative_base import new_program_scope
 
 
-class MyLayer(fluid.imperative.PyLayer):
+class MyLayer(fluid.imperative.Layer):
     def __init__(self):
         super(MyLayer, self).__init__()
 
@@ -34,7 +34,7 @@ class MyLayer(fluid.imperative.PyLayer):
         return [x]
 
 
-class MLP(fluid.imperative.PyLayer):
+class MLP(fluid.imperative.Layer):
     def __init__(self):
         super(MLP, self).__init__()
         self._fc1 = FC(3,
@@ -56,7 +56,7 @@ class TestImperative(unittest.TestCase):
         with fluid.imperative.guard():
             cl = core.Layer()
             cl.forward([])
-            l = fluid.imperative.PyLayer()
+            l = fluid.imperative.Layer()
             self.assertRaises(NotImplementedError, l.forward, [])
 
     def test_layer_in_out(self):

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

@@ -26,7 +26,7 @@ from paddle.fluid.imperative.base import to_variable
 from test_imperative_base import new_program_scope
 
 
-class SimpleImgConvPool(fluid.imperative.PyLayer):
+class SimpleImgConvPool(fluid.imperative.Layer):
     def __init__(self,
                  num_channels,
                  num_filters,
@@ -72,7 +72,7 @@ class SimpleImgConvPool(fluid.imperative.PyLayer):
         return x
 
 
-class MNIST(fluid.imperative.PyLayer):
+class MNIST(fluid.imperative.Layer):
     def __init__(self, param_attr=None, bias_attr=None):
         super(MNIST, self).__init__()