update GradientMachine API

paddle/api/GradientMachine.cpp

@@ -146,7 +146,7 @@ void GradientMachine::randParameters() { m->machine->randParameters(); }
 
 Matrix* GradientMachine::getLayerOutput(const std::string& layerName) const
     throw(UnsupportError) {
-  auto nn = std::dynamic_pointer_cast<paddle::NeuralNetwork>(m->machine);
+  auto nn = m->machine;
   if (nn) {
     auto mat = nn->getLayerOutput(layerName);
     return Matrix::createByPaddleMatrixPtr(&mat);