8 - RNN and LSTM

ucla | CS 162 | 2024-02-06 13:16

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Table of Contents

• RNNs
  • Long-Term Dependency Limitation
• LSTMs
  • Input Gate
  • Forget Gate
  • Next Step Context (Cell State)
  • Output Gate
  • Complete Structure
• Learning Neural LMs (Recap)

RNNs

• use sequential information to make preds
  • 
• don’t make independence assumption (unmlike FFNNs)
• perform same task at each step of a seq and inputs of next step req prev step outputs -> memory
• this is better than n-gram because we can generate probs conditioned on the WHOLE context $[1:t-1]$ while n-grams are usually constrained b/w $[t-n+1:t-1]$
• 

  Long-Term Dependency Limitation

• RNNs are theoretically capable of handling long term memory that generates outputs that are dependent on tokens much much earlier in a long context
• but irl they are not do to vanishing gradients of context from very far back in the context

  LSTMs

• designed to mitigate long-term dependency issue w/ RNNs
• the key is memory cell state that add or remove info as seq progresses by changing cascading multiplications to additions of probs
• this is done using 3 gates to control memory (input, forget, and NS/FS)
• NOTE: different weights across each gate (look at subscript)

  Input Gate

• decides what info from current input to capture in cell state
• consists of 2 parts
  • sigmoid (input gate layer) - decides what values to update b/w/ 0 to 1
  • tanh - creates a vector of new candidate values (contextualized seqs) ${\tilde{C}}_t$
• e.g., adding gender context of new subject to cell state and replace old context

  Forget Gate

• decides what info to remove through sigmoid layer
• looks at prev hidden state $h_{t-1}$ and current input $x_t$ and outptus vec bw 0 to 1 (1 = keep completely and v.v.)
• e.g., forget gender of old subject when we see new subject

  Next Step Context (Cell State)

• update old state $C_{t-1}$ into new cell state
• multiply old state by the forget gate $f_t$
• add in input gate $i_t\ast {\tilde{C}}_t$ (element-wise multiplication)

  Output Gate

• decide outputs by computing output gate then multiply into cell state to get outputs and propagate to next cell

  Complete Structure

• add bias to each of the below params

Learning Neural LMs (Recap)