Review — Adaptive Importance Sampling to Accelerate Training of a Neural Probabilistic Language Model

Feedforward Neural Network for Word Prediction

this story, Adaptive Importance Sampling to Accelerate Training of a Neural Probabilistic Language Model, by Université de Montréal, is reviewed. This is a paper by Prof. Yoshua Bengio. In this paper:

  • A feedforward neural network is trained to approximate probabilities over sequences of words.
  • Adaptive importance sampling is designed to accelerate the training.

This is a paper in 2007 TNN with over 200 citations, where TNN has become TNNLS in 2011, and TNNLS has high impact factor of 10.451. (Sik-Ho Tsang @ Medium) Though this paper mainly targets for predicting the next word, word prediction is the foundation to build a language model.

Outline

  1. Neural Language Model Architecture
  2. Adaptive Importance Sampling
  3. Experimental Results

1. Neural Language Model Architecture

Neural Language Model Architecture
  • Basically, the network we may think is simple if we compare with the current SOTA approach. Yet, it is amazing at that moment.
  • For the existing words wt-1 to wt-n+1, they are transformed to zi using the shared weight C.
  • For the next word that needs to predict, a separate D is used to transform it to z0.
  • Then, a hidden layer of W (weights) with d (bias) is used with tanh activation to transform z to a:
  • Finally, the output is a scalar energy function:
  • where bwt is bias and Vwt is the weight from hidden layer to output layer. To obtain the probability:
  • where:
  • i.e. the softmax operation.

2. Adaptive Importance Sampling

2.1. Classical Monte Carlo

  • At that moment, conventionally, classical Monte Carlo was used to estimate the gradient of the log-likelihood:

2.2. Biased Importance Sampling

  • In this paper, Biased Importance Sampling is proposed:
  • where a multiplicative constant w is used.
  • Thus, the gradient updated is scaled.
  • which is similar to nowadays weight update procedure.

2.3. Effective Sample Size (ESS)

  • That is similar to the minibatch size nowadays but ESS is adaptive to w:

3. Experimental Results

  • Brown corpus dataset is used.
  • The Brown corpus consists of 1,181,041 words from various American English documents.
  • The corpus was divided in train (800,000 words), validation (200,000 words), and test (the remaining 180,000 words) sets.
  • The vocabulary was truncated by mapping all “rare” words (words that appear three times or less in the corpus) into a single special word.
  • The resulting vocabulary contains 14,847 words.
  • A simple interpolated trigram, serving as baseline, achieves a perplexity of 253.8 on the test set.
Training error with respect to number of epochs
Validation and test errors with respect to CPU time
  • The figure shows that the convergence of both networks is similar. The same holds for validation and test errors.
  • The network trained by sampling converges to an even lower perplexity than the ordinary one (trained with the exact gradient).

After 9 epochs (26h), its perplexity over the test set is equivalent to that of the one trained with exact gradient at its overfitting point (18 epochs, 113 days).

  • Surprisingly enough, if letting the sampling-trained model converge, it starts to overfit at epoch 18 — as for classical training — but with a lower test perplexity of 196.6, a 3.8% improvement.
  • Total improvement in test perplexity with respect to the trigram baseline is 29%.
  • The required number of samples with the non-adaptive unigram was growing exponentially.

Reference

[2007 TNN] [Bengio TNN’07]
Adaptive Importance Sampling to Accelerate Training of a Neural Probabilistic Language Model

Natural Language Processing (NLP)

Language Model: 2007 [Bengio TNN’07]
Machine Translation: 2014
[Seq2Seq] [RNN Encoder-Decoder] 2015 [Attention Decoder/RNNSearch]
Image Captioning: 2015 [m-RNN] [R-CNN+BRNN] [Show and Tell/NIC]

My Other Previous Paper Readings

PhD, Researcher. I share what I've learnt and done. :) My LinkedIn: https://www.linkedin.com/in/sh-tsang/, My Paper Reading List: https://bit.ly/33TDhxG