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Neural Networks Luke Zettlemoyer (Slides adapted from Danqi Chen, - PowerPoint PPT Presentation

CSEP 517 Natural Language Processing Neural Networks Luke Zettlemoyer (Slides adapted from Danqi Chen, Chris Manning, Dan Jurafsky) Neural networks for NLP Feed-forward NNs Recurrent NNs Transformer Convolutional NNs Always coupled with


  1. CSEP 517 Natural Language Processing Neural Networks Luke Zettlemoyer (Slides adapted from Danqi Chen, Chris Manning, Dan Jurafsky)

  2. Neural networks for NLP Feed-forward NNs Recurrent NNs Transformer Convolutional NNs Always coupled with word embeddings…

  3. This Lecture • Feedforward Neural Networks • Applications • Neural Bag-of-Words Models • Feedforward Neural Language Models • The training algorithm: Back-propagation

  4. Neural Networks: History

  5. NN “dark ages” • Neural network algorithms date from the 80s • ConvNets: applied to MNIST by LeCun in 1998 • Long Short-term Memory Networks (LSTMs): Hochreiter and Schmidhuber 1997 • Henderson 2003: neural shift-reduce parser, not SOTA Credits: Greg Durrett

  6. 2008-2013: A glimmer of light • Collobert and Weston 2011: “ NLP (almost) from Scratch ” • Feedforward NNs can replace “feature engineering” • 2008 version was marred by bad experiments, claimed SOTA but wasn’t, 2011 version tied SOTA • Krizhevskey et al, 2012: AlexNet for ImageNet Classification • Socher 2011-2014: tree-structured RNNs working okay Credits: Greg Durrett

  7. 2014: Stuff starts working • Kim (2014) + Kalchbrenner et al, 2014: sentence classification • ConvNets work for NLP! • Sutskever et al, 2014: sequence-to-sequence for neural MT • LSTMs work for NLP! • Chen and Manning 2014: dependency parsing • Even feedforward networks work well for NLP! • 2015: explosion of neural networks for everything under the sun Credits: Greg Durrett

  8. Why didn’t they work before? • Datasets too small : for MT, not really better until you have 1M+ parallel sentences (and really need a lot more) • Optimization not well understood : good initialization, per- feature scaling + momentum (Adagrad/Adam) work best out-of- the-box • Regularization: dropout is pretty helpful • Computers not big enough: can’t run for enough iterations • Inputs: need word embeddings to represent continuous semantics Credits: Greg Durrett

  9. The “Promise” • Most NLP works in the past focused on human-designed representations and input features • Representation learning attempts to automatically learn good features and representations • Deep learning attempts to learn multiple levels of representation on increasing complexity/abstraction

  10. Feed-forward Neural Networks

  11. Feed-forward NNs • Input: x 1 , …, x d • Output: y ∈ {0,1}

  12. Neural computation Computation units: neurons

  13. An artificial neuron • A neuron is a computational unit that has scalar inputs and an output • Each input has an associated weight. • The neuron multiples each input by its weight, sums them, applied a nonlinear function to the result, and passes it to its output.

  14. Neural networks • The neurons are connected to each other, forming a network • The output of a neuron may feed into the inputs of other neurons

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  16. A neural network = many layers of classifiers all learned at once, some providing features for others • If we feed a vector of inputs through a bunch of logistic regression functions, then we get a vector of outputs… • which we can feed into another logistic regression function

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