A bi-LSTM layer works by applying two LSTM layers on the data; one in the forward direction and one in the reverse direction. You can apply an LSTM function in the reverse direction by flipping the data. The results from these two LSTM layers is then concatenated together to form the output of the bi-LSTM layer. So if we want to implement a bi-GRU layer, we can do this by using a custom flip layer together with GRU layers. A custom flip layer can be implemented as follows:
classdef FlipLayer < nnet.layer.Layer
methods
function layer = FlipLayer(name)
layer.Name = name;
end
function Y = predict(~, X)
Y = flip(X,3);
end
end
end
We can implement a bi-GRU layer with OutputMode="sequence" by arranging layers in the way shown below:
We can implement a bi-GRU layer with with OutputMode="last" by arranging layers in the way shown below:
Below is a short example showing how to use the custom flip layer mentioned above to implement a network with two bi-GRU layers (one with OutputMode="sequence" and oine with OutputMode="last"):
[XTrain, YTrain] = japaneseVowelsTrainData;
lg = layerGraph();
lg = addLayers(lg, [
sequenceInputLayer(12, "Name", "input")
gruLayer( 100, 'OutputMode', 'sequence', ...
"Name", "gru1")
concatenationLayer(1, 2, "Name", "cat1")
gruLayer( 100, 'OutputMode', 'last', ...
"Name", "gru3")
concatenationLayer(1, 2, "Name", "cat2")
fullyConnectedLayer(9)
softmaxLayer
classificationLayer()] );
lg = addLayers( lg, [
FlipLayer("flip1")
gruLayer( 100, 'OutputMode', 'sequence', ...
"Name", "gru2" )
FlipLayer("flip2")] );
lg = addLayers(lg, [
FlipLayer("flip3")
gruLayer( 100, 'OutputMode', 'last', ...
"Name", "gru4" )] );
lg = connectLayers(lg, "input", "flip1");
lg = connectLayers(lg, "flip2", "cat1/in2");
lg = connectLayers(lg, "cat1", "flip3");
lg = connectLayers(lg, "gru4", "cat2/in2");
options = trainingOptions('adam', 'Plots', 'training-progress');
net = trainNetwork(XTrain, YTrain, lg, options);
[XTest, YTest] = japaneseVowelsTestData;
YPred = classify(net, XTest);
accuracy = sum(YTest == YPred)/numel(YTest)
