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Long short-term memory (LSTM) [1] is a type of recurrent neural network (RNN) aimed at mitigating the vanishing gradient problem [2] commonly encountered by traditional RNNs. Its relative insensitivity to gap length is its advantage over other RNNs, hidden Markov models , and other sequence learning methods.
That is, LSTM can learn tasks that require memories of events that happened thousands or even millions of discrete time steps earlier. Problem-specific LSTM-like topologies can be evolved. [56] LSTM works even given long delays between significant events and can handle signals that mix low and high-frequency components.
An LSTM unit contains three gates: An input gate, which controls the flow of new information into the memory cell; A forget gate, which controls how much information is retained from the previous time step; An output gate, which controls how much information is passed to the next layer. The equations for LSTM are: [2]
For a concrete example, consider a typical recurrent network defined by = (,,) = + + where = (,) is the network parameter, is the sigmoid activation function [note 2], applied to each vector coordinate separately, and is the bias vector.
In an autoregressive task, [50] the entire sequence is masked at first, and the model produces a probability distribution for the first token. Then the first token is revealed and the model predicts the second token, and so on. The loss function for the task is still typically the same. The GPT series of models are trained by autoregressive tasks.
A key breakthrough was LSTM (1995), [note 1] a RNN which used various innovations to overcome the vanishing gradient problem, allowing efficient learning of long-sequence modelling. One key innovation was the use of an attention mechanism which used neurons that multiply the outputs of other neurons, so-called multiplicative units . [ 11 ]
Bidirectional recurrent neural networks (BRNN) connect two hidden layers of opposite directions to the same output.With this form of generative deep learning, the output layer can get information from past (backwards) and future (forward) states simultaneously.
There are four sources of uncertainty regarding predictions obtained in this manner: (1) uncertainty as to whether the autoregressive model is the correct model; (2) uncertainty about the accuracy of the forecasted values that are used as lagged values in the right side of the autoregressive equation; (3) uncertainty about the true values of ...