1. Create an output layer using OutputLayer:

OutputLayer outputLayer = new OutputLayer.Builder().nIn(250).nOut(784)
 .lossFunction(LossFunctions.LossFunction.MSE)
 .build();

2. Add OutputLayer to the layer definitions:

builder.layer(new OutputLayer.Builder().nIn(250).nOut(784)
 .lossFunction(LossFunctions.LossFunction.MSE)
 .build());