Merge pull request #15229 from panyx0718/imperative

support python codes in the imperative model

paddle/fluid/imperative/layer.cc

@@ -27,6 +27,8 @@
 namespace paddle {
 namespace imperative {
 
+std::map<int, py::object> py_funcs_;
+
 using framework::Variable;
 
 void AddTo(Variable* src, Variable* dst) {
@@ -55,6 +57,7 @@ class Autograd {
     if (var->stop_gradient_) {
       return;
     }
+    VLOG(3) << "start autograd";
 
     std::deque<OpBase*> ready;
     ready.push_back(var->pre_op_);
@@ -120,22 +123,24 @@ framework::LoDTensor& VarBase::Grad() {
 }
 
 std::map<std::string, std::vector<VarBase*>> OpBase::ApplyGrad() {
-  if (!grad_op_desc_) {
+  if (!grad_op_desc_ && backward_id_ <= 0) {
     LOG(WARNING) << "op with no grad: " << op_desc_->Type();
     return {};
   }
-  VLOG(3) << "op grad " << grad_op_desc_->Type();
 
-  std::vector<std::unique_ptr<framework::Variable>> tmp_vars;
   std::map<std::string, std::vector<framework::Variable*>> grad_outputs;
+  if (backward_id_ > 0) {
+    VLOG(3) << "py_layer_grad";
+    grad_outputs["Out@GRAD"] =
+        PyLayer::ApplyGrad(backward_id_, grad_input_vars_["X@GRAD"]);
+  } else {
+    VLOG(3) << "op grad " << grad_op_desc_->Type();
     for (auto it : grad_output_vars_) {
       auto& outputs = grad_outputs[it.first];
       for (size_t i = 0; i < it.second.size(); ++i) {
         // Allocate a new variable
         Variable* tmp_var = new framework::Variable();
         tmp_var->GetMutable<framework::LoDTensor>();
-
-      tmp_vars.emplace_back(tmp_var);
         outputs.push_back(tmp_var);
       }
     }
@@ -157,14 +162,18 @@ std::map<std::string, std::vector<VarBase*>> OpBase::ApplyGrad() {
     PreparedOp p = PreparedOp::Prepare(ctx, *op_kernel, place);
     p.op.RuntimeInferShape(scope, place, ctx);
     p.func(framework::ExecutionContext(p.op, scope, *p.dev_ctx, p.ctx));
+  }
 
   for (auto it : grad_output_vars_) {
     auto& outputs = grad_outputs[it.first];
     auto& origin_outputs = it.second;
+    PADDLE_ENFORCE_EQ(outputs.size(), origin_outputs.size());
 
     for (size_t i = 0; i < outputs.size(); ++i) {
+      framework::Variable* grad = outputs[i];
       framework::Variable* orig_grad = origin_outputs[i];
-      AddTo(outputs[i], orig_grad);
+      AddTo(grad, orig_grad);
+      delete grad;
     }
   }
   return input_vars_;
@@ -173,6 +182,7 @@ std::map<std::string, std::vector<VarBase*>> OpBase::ApplyGrad() {
 void VarBase::RunBackward() {
   if (!pre_op_) return;
 
+  VLOG(3) << "start backward";
   auto grads_t = grads_->GetMutable<framework::LoDTensor>();
   float* data = grads_t->mutable_data<float>(platform::CPUPlace());
   std::fill(data, data + grads_t->numel(), 1.0);
@@ -183,5 +193,65 @@ void VarBase::RunBackward() {
   Autograd().RunBackward(this);
 }
 
+void PyLayer::RegisterFunc(int func_id, const py::object& py_func) {
+  py_funcs_[func_id] = py_func;
+}
+
+int PyLayer::NumFuncs() { return py_funcs_.size(); }
+
+std::vector<VarBase*> PyLayer::Apply(int func_id,
+                                     const std::vector<VarBase*>& inputs) {
+  std::vector<framework::Variable*> invars;
+  for (const VarBase* in : inputs) {
+    invars.push_back(in->var_);
+  }
+  PADDLE_ENFORCE(py_funcs_.find(func_id) != py_funcs_.end());
+  std::vector<Variable*> outvars = CallPythonFunc(py_funcs_[func_id], invars);
+  std::vector<VarBase*> ret;
+  for (Variable* v : outvars) {
+    ret.push_back(new VarBase(v, new Variable()));
+  }
+  return ret;
+}
+
+std::vector<Variable*> PyLayer::ApplyGrad(
+    int func_id, const std::vector<framework::Variable*>& inputs) {
+  PADDLE_ENFORCE(py_funcs_.find(func_id) != py_funcs_.end());
+  return CallPythonFunc(py_funcs_[func_id], inputs);
+}
+
+std::vector<framework::Variable*> PyLayer::CallPythonFunc(
+    const py::object& callable, const std::vector<framework::Variable*>& ins) {
+  py::gil_scoped_acquire guard;
+  py::tuple in_args(ins.size());
+  for (size_t i = 0; i < ins.size(); ++i) {
+    const framework::LoDTensor& t = ins[i]->Get<framework::LoDTensor>();
+    in_args[i] = t.IsInitialized() ? py::cast(t) : py::cast(nullptr);
+  }
+  VLOG(3) << "pyfunc in " << py::len(in_args);
+
+  // TODO(panyx0718): Who owns the returned LoDTensor.
+  auto ret = callable(in_args);
+  auto ret_tuple = py::cast<py::tuple>(ret);
+  size_t ret_num = py::len(ret_tuple);
+  std::vector<framework::Variable*> outs;
+  VLOG(3) << "pyfunc out " << ret_num;
+  for (size_t i = 0; i < ret_num; ++i) {
+    try {
+      auto* py_out_tensor = py::cast<framework::LoDTensor*>(ret_tuple[i]);
+      PADDLE_ENFORCE_NOT_NULL(py_out_tensor,
+                              "Output tensor %d should not be nullptr", i);
+      auto* var = new framework::Variable();
+      auto* tensor = var->GetMutable<framework::LoDTensor>();
+      tensor->ShareDataWith(*py_out_tensor);
+      tensor->set_lod(py_out_tensor->lod());
+      outs.push_back(var);
+    } catch (py::cast_error&) {
+      PADDLE_THROW("The %d-th output must be LoDTensor", i);
+    }
+  }
+  return outs;
+}
+
 }  // namespace imperative
 }  // namespace paddle

paddle/fluid/imperative/layer.h

@@ -17,6 +17,9 @@
 #include <map>
 #include <string>
 #include <vector>
+#include "pybind11/pybind11.h"
+
+#include "Python.h"
 #include "paddle/fluid/framework/op_desc.h"
 #include "paddle/fluid/framework/operator.h"
 #include "paddle/fluid/framework/var_desc.h"
@@ -25,6 +28,8 @@
 namespace paddle {
 namespace imperative {
 
+namespace py = ::pybind11;
+
 class PreparedOp {
  public:
   PreparedOp(const framework::OperatorBase& op,
@@ -82,12 +87,15 @@ class OpBase;
 
 class VarBase {
  public:
-  VarBase()
+  VarBase() : VarBase(new framework::Variable(), new framework::Variable()) {}
+
+  // Owns `var` and `grad`
+  VarBase(framework::Variable* var, framework::Variable* grad)
       : pre_op_(nullptr),
         pre_op_out_idx_(-1),
         var_desc_(nullptr),
-        var_(new framework::Variable()),
-        grads_(new framework::Variable()),
+        var_(var),
+        grads_(grad),
         stop_gradient_(false) {}
 
   explicit VarBase(bool stop_gradient)
@@ -124,7 +132,11 @@ class VarBase {
 
 class OpBase {
  public:
-  OpBase() : op_desc_(nullptr), grad_op_desc_(nullptr) {}
+  OpBase()
+      : op_desc_(nullptr),
+        forward_id_(-1),
+        grad_op_desc_(nullptr),
+        backward_id_(-1) {}
 
   virtual ~OpBase() {
     if (grad_op_desc_) delete grad_op_desc_;
@@ -132,8 +144,14 @@ class OpBase {
 
   std::map<std::string, std::vector<VarBase*>> ApplyGrad();
 
+  // One of `op_desc_` or `forward_id_` is set, not both.
+  // For pure python PyLayer, use `forward_id_`, otherwise, use op_desc_.
   framework::OpDesc* op_desc_;
+  int forward_id_;
+  // When has backward, one of `grad_op_desc_` or `backward_id_` is set,
+  // not both.
   framework::OpDesc* grad_op_desc_;
+  int backward_id_;
 
   std::map<std::string, std::vector<VarBase*>> input_vars_;
   std::map<std::string, std::vector<VarBase*>> output_vars_;
@@ -153,8 +171,25 @@ class Layer {
     std::vector<VarBase> vars;
     return vars;
   }
+};
+
+class PyLayer {
+ public:
+  virtual ~PyLayer() {}
+
+  static void RegisterFunc(int func_id, const py::object& py_func);
+
+  static int NumFuncs();
+
+  static std::vector<VarBase*> Apply(int func_id,
+                                     const std::vector<VarBase*>& inputs);
+
+  static std::vector<framework::Variable*> ApplyGrad(
+      int func_id, const std::vector<framework::Variable*>& inputs);
 
-  virtual void Backward() { LOG(ERROR) << "To support customize"; }
+ private:
+  static std::vector<framework::Variable*> CallPythonFunc(
+      const py::object& callable, const std::vector<framework::Variable*>& ins);
 };
 
 }  // namespace imperative

paddle/fluid/imperative/tracer.h

@@ -131,8 +131,10 @@ class Tracer {
 
     if (!stop_gradient) {
       framework::OpDesc* grad_op_desc;
-      auto grad_to_var = new std::unordered_map<std::string, std::string>();
-      CreateGradOp(*op_desc, {}, {block}, &grad_op_desc, grad_to_var);
+      // TODO(panyx): Is this leaked?
+      std::unique_ptr<std::unordered_map<std::string, std::string>> grad_to_var(
+          new std::unordered_map<std::string, std::string>());
+      CreateGradOp(*op_desc, {}, {block}, &grad_op_desc, grad_to_var.get());
       op->grad_op_desc_ = grad_op_desc;
 
       for (auto it : grad_op_desc->Inputs()) {
@@ -143,12 +145,14 @@ class Tracer {
           if (var_it == grad_to_var->end()) {
             auto fwd_var_it = vars.find(grad_invar);
             PADDLE_ENFORCE(fwd_var_it != vars.end());
+            // Forward inputs or outputs.
             grad_in_vars.push_back(fwd_var_it->second->var_);
           } else {
             VarBase* var = vars[var_it->second];
             if (!var->grads_->IsInitialized()) {
               InitVar(var->var_, var->grads_);
             }
+            // Douts.
             grad_in_vars.push_back(var->grads_);
           }
         }
@@ -172,6 +176,54 @@ class Tracer {
     op->block_ = block;
   }
 
+  std::vector<VarBase*> PyTrace(OpBase* op, const std::vector<VarBase*>& inputs,
+                                bool stop_gradient = false) {
+    VLOG(3) << "py_trace";
+    op->input_vars_["X"] = inputs;
+    op->output_vars_["Out"] = PyLayer::Apply(op->forward_id_, inputs);
+    for (VarBase* inp : inputs) {
+      if (inp->pre_op_) {
+        op->pre_ops_["X"].push_back(inp->pre_op_);
+        op->pre_ops_out_idx_["X"].push_back(inp->pre_op_out_idx_);
+      } else {
+        op->pre_ops_["X"].push_back(nullptr);
+      }
+    }
+
+    auto& outputs = op->output_vars_["Out"];
+    for (size_t i = 0; i < outputs.size(); ++i) {
+      VarBase* out = outputs[i];
+      out->stop_gradient_ = stop_gradient;
+      out->pre_op_ = op;
+      out->pre_op_out_name_ = "Out";
+      out->pre_op_out_idx_ = i;
+    }
+    if (!stop_gradient) {
+      auto& grad_input_vars = op->grad_input_vars_["X@GRAD"];
+      auto& grad_output_vars = op->grad_output_vars_["Out@GRAD"];
+
+      for (const VarBase* inp : inputs) {
+        grad_input_vars.push_back(inp->var_);
+      }
+      for (VarBase* out : outputs) {
+        grad_input_vars.push_back(out->var_);
+      }
+      for (VarBase* out : outputs) {
+        grad_input_vars.push_back(out->grads_);
+        if (!grad_input_vars.back()->IsInitialized()) {
+          InitVar(out->var_, grad_input_vars.back());
+        }
+      }
+      for (const VarBase* inp : inputs) {
+        grad_output_vars.push_back(inp->grads_);
+        if (!grad_output_vars.back()->IsInitialized()) {
+          InitVar(inp->var_, grad_output_vars.back());
+        }
+      }
+    }
+    return outputs;
+  }
+
  private:
   framework::BlockDesc* root_block_;
 };

paddle/fluid/pybind/imperative.cc

@@ -26,7 +26,9 @@ void BindTracer(pybind11::module *m) {
            [](imperative::Tracer &self, framework::BlockDesc *root_block) {
              new (&self) imperative::Tracer(root_block);
            })
-      .def("trace", &imperative::Tracer::Trace);
+      .def("trace", &imperative::Tracer::Trace)
+      .def("py_trace", &imperative::Tracer::PyTrace,
+           pybind11::return_value_policy::take_ownership);
 }
 
 }  // namespace pybind

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
 
@@ -31,10 +31,6 @@ class PyLayer : public imperative::Layer {
     PYBIND11_OVERLOAD(std::vector<imperative::VarBase>, Layer, Forward,
                       inputs);  // NOLINT
   }
-
-  void Backward() override {
-    PYBIND11_OVERLOAD(void, Layer, Backward, );  // NOLINT
-  }
 };
 
 class PyOpBase : public imperative::OpBase {

paddle/fluid/pybind/pybind.cc

@@ -169,16 +169,44 @@ PYBIND11_MODULE(core, m) {
               self.op_desc_ = op_desc;
             }
           },
+          py::return_value_policy::reference)
+      .def_property(
+          "forward_id",
+          [](const imperative::OpBase &self) { return self.forward_id_; },
+          [](imperative::OpBase &self, int forward_id) {
+            self.forward_id_ = forward_id;
+          },
+          py::return_value_policy::reference)
+      .def_property(
+          "backward_id",
+          [](const imperative::OpBase &self) { return self.backward_id_; },
+          [](imperative::OpBase &self, int backward_id) {
+            self.backward_id_ = backward_id;
+          },
           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,
+      .def("forward", [](imperative::Layer &self,
                          const std::vector<imperative::VarBase> &inputs) {
         return self.Forward(inputs);
+      });
+
+  py::class_<imperative::PyLayer>(m, "PyLayer")
+      .def(py::init<>())
+      .def_static(
+          "apply",
+          [](int func_id, const std::vector<imperative::VarBase *> &inputs)
+              -> std::vector<imperative::VarBase *> {
+                return imperative::PyLayer::Apply(func_id, inputs);
+              },
+          py::return_value_policy::take_ownership)
+      .def_static("register_func",
+                  [](int func_id, const py::object &callable) {
+                    imperative::PyLayer::RegisterFunc(func_id, callable);
                   })
-      .def("backward", &imperative::Layer::Backward);
+      .def_static("num_funcs", &imperative::PyLayer::NumFuncs);
+
   BindTracer(&m);
 
   py::class_<Tensor>(m, "Tensor", py::buffer_protocol())

python/paddle/fluid/framework.py

@@ -372,6 +372,9 @@ class Variable(object):
         self.stop_gradient = stop_gradient
         self.is_data = is_data
         if _in_imperative_mode():
+            if 'ivar' in kwargs:
+                self._ivar = kwargs['ivar']
+            else:
                 self._ivar = core.VarBase()
             self._ivar.desc = self.desc
             self._ivar.stop_gradient = stop_gradient

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', 'PyLayer']
 
 
-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 +39,56 @@ 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.PyLayer):
+    """Layers composed of user-defined python codes."""
+
+    def __init__(self):
+        super(PyLayer, self).__init__()
+
+    @staticmethod
+    def forward(inputs):
+        raise NotImplementedError
+
+    @staticmethod
+    def backward(douts):
+        raise NotImplementedError
+
+    @classmethod
+    def __call__(cls, inputs):
+        tracer = framework._imperative_tracer()
+        block = framework.default_main_program().current_block()
+        inputs = [x._ivar for x in inputs]
+
+        if not hasattr(cls, 'forward_id'):
+            cls.forward_id = core.PyLayer.num_funcs() + 1
+            PyLayer.register_func(cls.forward_id, cls.forward)
+            cls.backward_id = core.PyLayer.num_funcs() + 1
+            PyLayer.register_func(cls.backward_id, cls.backward)
+
+        iop = core.OpBase()
+        iop.forward_id = cls.forward_id
+        iop.backward_id = cls.backward_id
+        block.ops.append(iop)
+        ivars = tracer.py_trace(iop, inputs, False)
+        # ivars = core.PyLayer.apply(cls.forward, inputs)
+        ret = []
+        for ivar in ivars:
+            tensor = ivar.value.get_tensor()
+            py_var = framework.Variable(
+                block,
+                type=core.VarDesc.VarType.LOD_TENSOR,
+                name=None,
+                shape=tensor.shape(),
+                dtype=tensor._dtype(),
+                ivar=ivar)
+            ret.append(py_var)
+        return ret

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

@@ -15,6 +15,7 @@
 import contextlib
 import unittest
 import numpy as np
+import sys
 
 import paddle.fluid as fluid
 from paddle.fluid import core
@@ -22,7 +23,7 @@ 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 +35,35 @@ class MyLayer(fluid.imperative.PyLayer):
         return [x]
 
 
-class MLP(fluid.imperative.PyLayer):
+class MyPyLayer(fluid.imperative.PyLayer):
+    def __init__(self):
+        super(MyPyLayer, self).__init__()
+
+    @staticmethod
+    def forward(inputs):
+        sys.stderr.write('before forward\n')
+        ret = np.tanh(inputs[0])
+        sys.stderr.write('after forward: %s\n' % ret)
+        tensor = core.LoDTensor()
+        tensor.set(ret, core.CPUPlace())
+        return tuple([tensor])
+
+    @staticmethod
+    def backward(inputs):
+        sys.stderr.write('calling into backward: %s\n' % str(inputs))
+        inp, out, dout = inputs
+        inp = np.array(inp)
+        out = np.array(out)
+        dout = np.array(dout)
+        sys.stderr.write('calling into backward: %s, %s, %s\n' %
+                         (inp, out, dout))
+        ret = np.array(dout) * (1 - np.square(np.array(out)))
+        tensor = core.LoDTensor()
+        tensor.set(ret, core.CPUPlace())
+        return tuple([tensor])
+
+
+class MLP(fluid.imperative.Layer):
     def __init__(self):
         super(MLP, self).__init__()
         self._fc1 = FC(3,
@@ -56,9 +85,77 @@ 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_pylayer_func_id(self):
+
+        with fluid.imperative.guard():
+
+            class PyLayer1(fluid.imperative.PyLayer):
+                def __init__(self):
+                    super(PyLayer1, self).__init__()
+
+                @staticmethod
+                def forward(inputs):
+                    return inputs
+
+                @staticmethod
+                def backward(inputs):
+                    return inputs
+
+            class PyLayer2(fluid.imperative.PyLayer):
+                def __init__(self):
+                    super(PyLayer2, self).__init__()
+
+                @staticmethod
+                def forward(inputs):
+                    return inputs
+
+                @staticmethod
+                def backward(inputs):
+                    return inputs
+
+            py_layer_1 = PyLayer1()
+            py_layer_2 = PyLayer2()
+            py_layer_1([fluid.imperative.base.to_variable(np.ones([2, 2]))])
+            py_layer_2([fluid.imperative.base.to_variable(np.ones([2, 2]))])
+            id = py_layer_1.forward_id
+            self.assertGreater(id, 0)
+            self.assertEqual(py_layer_1.backward_id, id + 1)
+            self.assertEqual(py_layer_2.forward_id, id + 2)
+            self.assertEqual(py_layer_2.backward_id, id + 3)
+            py_layer_1([fluid.imperative.base.to_variable(np.ones([2, 2]))])
+            self.assertEqual(py_layer_1.forward_id, id)
+
+    def test_pylayer(self):
+        np_inp = np.ones([2, 2], np.float32)
+        with fluid.imperative.guard():
+            my_py_layer = MyPyLayer()
+            var_inp = fluid.imperative.base.to_variable(np_inp)
+            outs = my_py_layer([var_inp])
+            dy_out = np.sum(outs[0]._numpy())
+            outs[0]._backward()
+            dy_grad = var_inp._gradient()
+
+        with new_program_scope():
+            inp = fluid.layers.data(
+                name="inp", shape=[2, 2], append_batch_size=False)
+            # TODO(panyx0718): Paddle doesn't diff against data `inp`.
+            x1 = inp * 1
+            # TODO(panyx0718): If reduce_sum is skipped, the result is wrong.
+            x = fluid.layers.reduce_sum(fluid.layers.tanh(x1))
+            param_grads = fluid.backward.append_backward(
+                x, parameter_list=[x1.name])[0]
+            exe = fluid.Executor(fluid.CPUPlace())
+
+            static_out, static_grad = exe.run(
+                feed={inp.name: np_inp},
+                fetch_list=[x.name, param_grads[1].name])
+
+        self.assertTrue(np.allclose(dy_out, static_out))
+        self.assertTrue(np.allclose(dy_grad, static_grad))
+
     def test_layer_in_out(self):
         np_inp = np.array([1.0, 2.0, -1.0], dtype=np.float32)
         with fluid.imperative.guard():

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__()