Add const for Trainer::getLayerOutput

paddle/api/PaddleAPI.h

@@ -953,7 +953,7 @@ public:
 
   Arguments* getForwardOutput();
 
-  Arguments* getLayerOutput(const std::string& layerName);
+  Arguments* getLayerOutput(const std::string& layerName) const;
 };
 
 /// the N-Best results generated from one input sequence.

paddle/api/Trainer.cpp

@@ -131,7 +131,7 @@ void Trainer::testOneDataBatch(size_t batchSize, const Arguments& args) {
 void TrainerPrivate::finishTestPeriod() { tester_->finishTestPeriod(); }
 void Trainer::finishTestPeriod() { m->finishTestPeriod(); }
 
-Arguments* Trainer::getLayerOutput(const std::string& layerName) {
+Arguments* Trainer::getLayerOutput(const std::string& layerName) const {
   auto nn = this->m->getGradientMachine();
   CHECK(nn) << "trainerInternal_.getGradientMachine() is not NeuralNetwork";
   return Arguments::createByPaddleArgument(&nn->getLayerOutput(layerName));