/// <summary>
/// Creates a LSTM cell that implements one repetition step in a recursive network.
/// It takes the previous state of the cell (c - cell state) and output (h - hidden state) as arguments.
/// Returns a tuple of the new state of the cell (c - cell state) and exit (h - hidden state).
/// </summary>
/// <param name="input">Entrance to LSTM (X at step t)</param>
/// <param name="prevOutput">The previous state of the output LSTM (h at step t-1)</param>
/// <param name="prevCellState">The previous state of the LSTM cell (s in step t-1)</param>
/// <param name="useShortcutConnections">Specifies whether to create ShortcutConnections for this cell.</param>
/// <param name="selfStabilizerLayer">Self-stabilization layer to the prevOutput and prevCellState inputs</param>
/// <param name="device">Device for calculations</param>
/// <returns></returns>
private static Tuple <Function, Function> LSTMCell(Variable input, Variable prevOutput,
Variable prevCellState, bool useShortcutConnections, Layer selfStabilizerLayer, DeviceDescriptor device)
{
int lstmOutputDimension = prevOutput.Shape[0];
int lstmCellDimension = prevCellState.Shape[0];
bool hasDifferentOutputAndCellDimension = lstmCellDimension != lstmOutputDimension;
DataType dataType = input.DataType;
if (selfStabilizerLayer != null)
{
prevOutput = selfStabilizerLayer.Create(prevOutput, device);
prevCellState = selfStabilizerLayer.Create(prevCellState, device);
}
uint seed = CNTKLib.GetRandomSeed();
//create an input projection of data from the input X [t] and the hidden state H [t-1]
Func <int, Variable> createInput = (outputDim) =>
{
var inputWeigths = new Parameter(new[] { outputDim, NDShape.InferredDimension }, dataType, CNTKLib.GlorotUniformInitializer(1.0, 1, 0, seed++), device);
var inputBias = new Parameter(new[] { outputDim }, dataType, CNTKLib.GlorotUniformInitializer(1.0, 1, 0, seed++), device);
var inputToCell = CNTKLib.Times(inputWeigths, input) + inputBias;
var gateInput = CNTKLib.Plus(inputToCell, prevOutput);
return(gateInput);
};
Func <int, Variable, Variable> createProjection = (targetDim, variableNeedsToProjection) =>
{
var cellWeigths = new Parameter(new[] { targetDim, NDShape.InferredDimension }, dataType, CNTKLib.GlorotUniformInitializer(1.0, 1, 0, seed++), device);
var projection = CNTKLib.Times(cellWeigths, variableNeedsToProjection);
return(projection);
};
Variable forgetProjection = createInput(lstmOutputDimension);
Variable inputProjection = createInput(lstmOutputDimension);
Variable candidateProjection = createInput(lstmOutputDimension);
Variable outputProjection = createInput(lstmOutputDimension);
Function forgetGate = CNTKLib.Sigmoid(forgetProjection); // forget valve (from the input in step t)
Function inputGate = CNTKLib.Sigmoid(inputProjection); //input gate (from the input in step t)
Function candidateGate = CNTKLib.Tanh(candidateProjection); //the candidate selection gate for storing in the cellular state (from the input in step t)
Function outputGate = CNTKLib.Sigmoid(outputProjection); //output gate (from the input in step t)
forgetGate = hasDifferentOutputAndCellDimension ? createProjection(lstmCellDimension, forgetGate) : (Variable)forgetGate;
Function forgetState = CNTKLib.ElementTimes(prevCellState, forgetGate); //forget what you need to forget in the cellular state
Function inputState = CNTKLib.ElementTimes(inputGate, candidateProjection); //we get what we need to save in the cellular state (from the input in step t)
inputState = hasDifferentOutputAndCellDimension ? createProjection(lstmCellDimension, inputState) : (Variable)inputState;
Function cellState = CNTKLib.Plus(forgetState, inputState); //add new information to the cellular state
Variable cellToOutputProjection = hasDifferentOutputAndCellDimension ? createProjection(lstmOutputDimension, cellState) : (Variable)cellState;
Function h = CNTKLib.ElementTimes(outputGate, CNTKLib.Tanh(cellToOutputProjection)); //get exit / hidden state
Function c = cellState;
if (useShortcutConnections)
{
var forwarding = input;
var inputDim = input.Shape[0];
if (inputDim != lstmOutputDimension)
{
var scales = new Parameter(new[] { lstmOutputDimension, inputDim }, dataType, CNTKLib.UniformInitializer(seed++), device);
forwarding = CNTKLib.Times(scales, input);
}
h = CNTKLib.Plus(h, forwarding);
}
return(new Tuple <Function, Function>(h, c));
}
/// <summary>
/// Creates an LSTM layer.
/// The cellular state (C) has an independent dimension, all gates have an output dimension (H), scaling is performed directly when writing to the cellular state.
/// The input (X [t]) is scaled to the output (H [t-1]) and summed (X [t] + H [t-1]), the memory cell (C) is scaled to the output (H).
/// </summary>
/// <param name="input">Entrance (X)</param>
/// <param name="lstmDimension">Output layer depth (H)</param>
/// <param name="cellDimension">Bit depth of the inner layer of the memory cell, if 0 - sets the bit depth of the output layer (C)</param>
/// <param name="useShortcutConnections">If true, use input forwarding parallel to the layer. Enabled by default.</param>
/// <param name="selfStabilizerLayer">Self-stabilization layer</param>
/// <param name="isLastLstm">Indicates whether this will be the last of the LSTM layers (the next layers on the network are non-recursive). In order to join LSTM layers one after another, all layers except the last one must be set to false</param>
/// <param name="outputName">layer name</param>
/// <returns></returns>
public static Function Build(Function input, int lstmDimension, DeviceDescriptor device, int cellDimension = 0, bool useShortcutConnections = true, bool isLastLstm = true, Layer selfStabilizerLayer = null, string outputName = "")
{
if (cellDimension == 0)
{
cellDimension = lstmDimension;
}
Func <Variable, Function> pastValueRecurrenceHook = (x) => CNTKLib.PastValue(x);
var lstm = LSTMComponent(input, new int[] { lstmDimension }, new int[] { cellDimension },
pastValueRecurrenceHook, pastValueRecurrenceHook, useShortcutConnections, selfStabilizerLayer, device)
.Item1;
lstm = isLastLstm ? CNTKLib.SequenceLast(lstm) : lstm;
return(Function.Alias(lstm, outputName));
}
/// <summary>
/// Creates an LSTM layer.
/// The cellular state (C) has an independent dimension, all gates have an output dimension (H), scaling is performed directly when writing to the cellular state.
/// The input (X [t]) is scaled to the output (H [t-1]), the memory cell (C) is scaled to the output (H).
/// </summary>
/// <param name="lstmOutputDim">Output layer depth (H)</param>
/// <param name="lstmCellDim">Bit depth of the inner layer of the memory cell, if 0 - sets the bit depth of the output layer (C)</param>
/// <param name="useShortcutConnections">If true, use input forwarding parallel to the layer. Enabled by default.</param>
/// <param name="selfStabilizerLayer">Self-stabilization layer</param>
/// <param name="isLastLstm">Indicates whether this will be the last of the LSTM layers (the next layers on the network are non-recursive). In order to join LSTM layers one after another, all layers except the last one must be set to false</param>
/// <param name="name"></param>
public LSTMv1(int lstmOutputDim, int lstmCellDim = 0, bool useShortcutConnections = true, bool isLastLstm = true, Layer selfStabilizerLayer = null, string name = "LSTMv1")
{
_lstmOutputDim = lstmOutputDim;
_lstmCellDim = lstmCellDim == 0 ? _lstmOutputDim : _lstmCellDim;;
_useShortcutConnections = useShortcutConnections;
_isLastLstmLayer = isLastLstm;
_selfStabilizerLayer = selfStabilizerLayer;
_name = name;
}
private static Tuple <Function, Function> LSTMComponent(Variable input, NDShape outputShape,
NDShape cellShape, Func <Variable, Function> recurrenceHookH, Func <Variable, Function> recurrenceHookC,
bool useShortcutConnections, Layer selfStabilizerLayer, DeviceDescriptor device)
{
var dh = Variable.PlaceholderVariable(outputShape, input.DynamicAxes);
var dc = Variable.PlaceholderVariable(cellShape, input.DynamicAxes);
var lstmCell = LSTMCell(input, dh, dc, useShortcutConnections, selfStabilizerLayer, device);
var actualDh = recurrenceHookH(lstmCell.Item1);
var actualDc = recurrenceHookC(lstmCell.Item2);
(lstmCell.Item1).ReplacePlaceholders(new Dictionary <Variable, Variable> {
{ dh, actualDh }, { dc, actualDc }
});
return(new Tuple <Function, Function>(lstmCell.Item1, lstmCell.Item2));
}
