stem/AI/Neural Networks/Transformers/Attention.md

• Meant to mimic cognitive attention
  • Picks out relevant bits of information
  • Use gradient descent
• Used in 90s
  • Multiplicative modules
  • Sigma pi units
  • Hyper-networks
• Draw from relevant state at any preceding point along sequence
  • Addresses RNNs vanishing gradient issues
  • LSTM tends to poorly preserve far back knowledge
• Attention layer access all previous states and weighs according to learned measure of relevance
  • Allows referring arbitrarily far back to relevant tokens
• Can be addd to RNNs
• In 2016, a new type of highly parallelisable decomposable attention was successfully combined with a feedforward network
  • Attention useful in of itself, not just with RNNs
• Transformers use attention without recurrent connections
  • Process all tokens simultaneously
  • Calculate attention weights in successive layers

Scaled Dot-Product

• Calculate attention weights between all tokens at once
• Learn 3 weight matrices
  • Query
    • W_Q
  • Key
    • W_K
  • Value
    • W_V
• Word vectors
  • For each token, i, input word embedding, x_i
    • Multiply with each of above to produce vector
  • Query Vector
    • q_i=x_iW_Q
  • Key Vector
    • k_i=x_iW_K
  • Value Vector
    • v_i=x_iW_V
• Attention vector
  • Query and key vectors between token i and j
  • a_{ij}=q_i\cdot k_j
  • Divided by root of dimensionality of key vectors
    • \sqrt{d_k}
  • Pass through softmax to normalise
• W_Q and W_K are different matrices
  • Attention can be non-symmetric
  • Token i attends to j (q_i\cdot k_j is large)
    • Doesn't imply that j attends to i (q_j\cdot k_i can be small)
• Output for token i is weighted sum of value vectors of all tokens weighted by a_{ij}
  • Attention from token i to each other token
• Q, K, V are matrices where $i$th row are vectors q_i, k_i, v_i respectively

\text{Attention}(Q,K,V)=\text{softmax}\left( \frac{QK^T}{\sqrt{d_k}} \right)V

• softmax taken over horizontal axis