Language is backend dependent

@svetlana.pavlitskaya @carlos.alfredo.yeverino

The generator is backend dependent now, i.e. the language contains an mxnet generator.

Since we want to extend the backend support to more backends, the train generator should not reside in the TrainLang project any more.

Please keep an abstract generator interface in CNNTrainLang and move the MXNetTrain generator to CNNArch2MxNet