Support for multiple classifiers

decoder/src/Beam.cpp

decoder/src/Decoder.cpp

decoder/src/MacaonDecode.cpp

reading_machine/include/Classifier.hpp

reading_machine/include/ReadingMachine.hpp

reading_machine/src/Classifier.cpp

reading_machine/src/ReadingMachine.cpp

torch_modules/src/DictHolder.cpp

trainer/src/MacaonTrain.cpp

trainer/src/Trainer.cpp