Encoder-Decoder Framework

Updated 2023-10-27

The Encoder-Decoder Framework is commonly used for solving sequence-to-sequence tasks, where it takes an input sequence, processes it through an encoder, and produces an output sequence. This framework consists of three main components: an encoder, a context vector, and a decoder, as illustrated in Figure 1:

Figure 1 - An overview of an encoder-decoder framework.

Encoder

An encoder processes an input sequence and creates a context vector that captures context from the entire sequence and serves as a summary of the input.

Let $X =[x_1, \ldots, x_n,x_c]$ be an input sequence, where $x_i \in \mathbb{R}^{d \times 1}$ is the $i$'th word in the sequence and $x_c$is an artificial token appended to indicate the end of the sequence. The encoder utilizes two functions, $f$ and $g$, which are defined in the same way as in the RNN for text classification. Notice that the end-of-sequence token $x_c$ is used to create an additional hidden state $h_c$, which in turn creates the context vector $y_c$.

Figure 2 shows an encoder example that takes the input sequence, "I am a boy", appended with the end-of-sequence token "[EOS]":

Figure 2 - An encoder example.

Is it possible to derive the context vector from $x_n$ instead of $x_c$? What is the purpose of appending an extra token to indicate the end of the sequence?

Decoder

A decoder is conditioned on the context vector, which allows it to generate an output sequence contextually relevant to the input, often one token at a time.

Let $Y = [y_1, \ldots, y_m, y_{t}]$ be an output sequence, where $y_i \in \mathbb{R}^{o \times 1}$ is the $i$'th word in the sequence, and $y_t$ is an artificial token to indicate the end of the sequence. To generate the output sequence, the decoder defines two functions, $f$ and $g$ :

• $f$ takes the previous output $y_{i-1}$ and its hidden state $s_{i-1}$, and returns a hidden state $s_{i} \in \mathbb{R}^{e \times 1}$ such that $f(y_{i-1}, s_{i-1}) = \alpha(W^y y_{i-1}​ + W^s s_ {i−1}) = s_i$, where $W^y \in \mathbb{R}^{e \times o}$, $W^s \in \mathbb{R}^{e \times e}$, and $\alpha$ is an activation function.
• $g$ takes the hidden state $s_i$ and returns an output $y_{i} \in \mathbb{R}^{o \times 1}$ such that $g(s_i) = W^o s_i = y_{i}$, where $W^o \in \mathbb{R}^{o \times e}$.

Note that the initial hidden state $s_1$ is created by considering only the context vector $y_c$such that the first output $y_1$ is solely predicted by the context in the input sequence. However, the prediction of every subsequent output $y_{i}$ is conditioned on both the previous output $y_{i-1}$ and its hidden state $s_{i-1}$. Finally, the decoder stops generating output when it predicts the end-of-sequence token $y_t$.

In some variations of the decoder, the initial hidden state $s_1$ is created by considering both $y_c$ and $h_c$ [1].

Figure 3 illustrates a decoder example that takes the context vector $y_0$ and generates the output sequence, "나(I) +는(SBJ) 소년(boy) +이다(am)", terminated by the end-of-sequence token "[EOS]", which translates the input sequence from English to Korean:

Figure 3 - A decoder example.

The decoder mentioned above does not guarantee the generation of the end-of-sequence token at any step. What potential issues can arise from this?

The likelihood of the current output $y_i$ can be calculated as:

$$P(y_{i} | \{y_c\} \cup \{y_1, \ldots, y_{i-1}\}) = q(y_c, y_{i-1}, s_{i-1})$$

where $q$ is a function that takes the context vector $y_c$, previous input $y_{i-1}$ and its hidden state $s_{i-1}$, and returns the probability of $y_i$. Then, the maximum likelihood of the output sequence can be estimated as follows ($y_0 = s_0 = \mathbf{0}$):

$$P(Y) = \prod_{i=1}^{m+1} q(y_c, y_{i-1}, s_{i-1})$$

The maximum likelihood estimation of the output sequence above accounts for the end-of-sequence token $y_t$. What are the benefits of incorporating this artificial token when estimating the sequence probability?

References

1. Sequence to Sequence Learning with Neural Networks, Sutskever et al., NeurIPS, 2014.*
2. Neural Machine Translation by Jointly Learning to Align and Translate, Bahdanau et al., ICLR, 2015.*