NLP Notes

Attention

Additive/concat Attention

• Resources: [Paper],  [Illustrative Intro],  [TF2 Implementation]

Multiplicative Attention

• Resources: [Paper],  [Illustrative Intro],  [TF2 Implementation]

Multi-head Self Attention / Transformer

• Resources: [Paper],  [Illustrative Intro],  [TF2 Implementation],  [TF2 Implementation by Google]

Subword Tokenization

• Summary: HuggingFace Tokenizer Summary
• Implementation: HuggingFace Tokenizer, Google SentencePiece
• Unigram Language Model (ULM)
  • assume all subword occurence are independent and subword sequence is produced by the product of subword occurrence probabilities
  • optimize for whole sentence likelihood probability (Viterbi Algorithm)
  • both WP and ULM leverages language model to build subword vocabulary
• Byte Pair Encoding (BPE)
  • start from character level, form a new subword based on the next highest frequency pair until reaching desired vocabulary size or the next highest frequency is 1
  • used in GPT-2, RoBERTa, see Git Issue for implementation
  • tokenizers.CharBPETokenizer: OpenAIGPTTokenizerFast,
  • tokenizers.ByteLevelBPETokenizer: GPT2TokenizerFast, RobertaTokenizerFast, LongformerTokenizerFast
• WordPiece (WP)
  • similar to BPE but "choose the new word unit out of all possible ones that increase the likelihood on the training data the most when added to the model"
    • define log P(sentence) = Σ log P(token_i)
      when merge adjacent tokens x and y into z
      the change in likelihood is log P(token_z) - (log P(token_x) + log P(token_y))
  • tokenizers.BertWordPieceTokenizer: BertTokenizerFast, DistilBertTokenizerFast, ElectraTokenizerFast, RetriBertTokenizerFast, MobileBertTokenizerFast

Industrial Application

Google Neural Machine Translation System

Concept applied: Additive/concat attention, Residual connection, Vanilla dropout
Resources: [Paper],  [Illustrative Intro],  [TF2 Implementation],  [Torch Implementation]

BERT: Bidirectional Encoder Representations from Transformers

Resources: [Paper]

Probabilistic Graph

Conditional Random Field

Resources: [Introduction to CRF],  [CRF vs MRF],  [CRF for Multi-label Classification]  [Tensorflow CRF];

Bi-LSTM CRF

Resources: [Paper],  [TF1.0 Implementation by Scofield]

Label Attention Network

Resources: [Paper],  [Torch Implementation by Author]

Modeling Tricks

Transformer Training

Pre-Layer Normalization Transformer: [Paper]
Training Tips for Transformer: [Paper]

Recurrent Neural Network Normalization

Resources: [Methodology Overview],  [Layer Normalization]
Experience: use BatchNormalization or LayerNormalization after each RNN layer

Recurrent Neural Network Dropout

Resources: [Methodology Overview],  [Vanilla Dropout],  [Variational Dropout],  [Recurrent Dropout]
Experience: set dropout ratio between 0.1 and 0.3, begin with vanilla dropout