Attention mechanisms revolutionized machine learning in applications ranging from NLP through computer vision to reinforcement learning. Attention is the key innovation behind the recent success of Transformer-based language models such as BERT.1 In this blog post, I will look at a first instance of attention that sparked the revolution - additive attention (also known as Bahdanau attention) proposed by Bahdanau et al.2
The idea of attention is quite simple: it boils down to weighted averaging. Let us consider machine translation as an example. When generating a translation of a source text, we first pass the source text through an encoder (an LSTM or an equivalent model) to obtain a sequence of encoder hidden states . Then, at each step of generating a translation (decoding), we selectively attend to these encoder hidden states, that is, we construct a context vector that is a weighted average of encoder hidden states:
We choose the weights based both on encoder hidden states and decoder hidden states and normalize them so that they encode a categorical probability distribution .
Intuitively, this corresponds to assigning each word of a source sentence (encoded as ) a weight that tells how much the word encoded by is relevant for generating subsequent th word (based on ) of a translation. The weighting function (also known as alignment function or score function) is responsible for this credit assignment.
There are many possible implementations of , including multiplicative (Luong) attention or key-value attention. In this blog post, I focus on the historically first and arguably the simplest one — additive attention.
Additive attention uses a single-layer feedforward neural network with hyperbolic tangent nonlinearity to compute the weights :
where and are matrices corresponding to the linear layer and is a scaling factor.
PyTorch Implementation of Additive Attention
class AdditiveAttention(torch.nn.Module): def __init__(self, encoder_dim=100, decoder_dim=50): super().__init__() self.encoder_dim = encoder_dim self.decoder_dim = decoder_dim self.v = torch.nn.Parameter(torch.rand(self.decoder_dim)) self.W_1 = torch.nn.Linear(self.decoder_dim, self.decoder_dim) self.W_2 = torch.nn.Linear(self.encoder_dim, self.decoder_dim) def forward(self, query, # [decoder_dim] values # [seq_length, encoder_dim] ): weights = self._get_weights(query, values) # [seq_length] weights = torch.nn.functional.softmax(weights, dim=0) return weights @ values # [encoder_dim] def _get_weights(self, query, # [decoder_dim] values # [seq_length, encoder_dim] ): query = query.repeat(values.size(0), 1) # [seq_length, decoder_dim] weights = self.W_1(query) + self.W_2(values) # [seq_length, decoder_dim] return torch.tanh(weights) @ self.v # [seq_length]
_get_weights corresponds to
query is a decoder hidden state
values is a matrix of encoder hidden states
. To keep the illustration clean, I ignore the batch dimension.
In practice, the attention mechanism handles queries at each time step of text generation.
context_vector corresponds to
c are LSTM’s hidden states, not crucial for our present purposes.
Finally, it is now trivial to access the attention weights and plot a nice heatmap.
attention = AdditiveAttention(encoder_dim=100, decoder_dim=50) encoder_hidden_states = torch.rand(10, 100) decoder_hidden_states = torch.rand(13, 50) weights = torch.FloatTensor(13, 10) for step in range(decoder_hidden_states.size(0)): context_vector = attention(decoder_hidden_states[step], encoder_hidden_states) weights[step] = attention._get_weights(decoder_hidden_states[step], encoder_hidden_states) seaborn.heatmap(weights.detach().numpy())
Here each cell corresponds to a particular attention weight . For a trained model and meaningful inputs, we could observe patterns there, such as those reported by Bahdanau et al. — the model learning the order of compound nouns (nouns paired with adjectives) in English and French. Let me end with this illustration of the capabilities of additive attention.
- Jacob Devlin, Ming-Wei Chang, Kenton Lee and Kristina Toutanova (2019). BERT: Pre-training of deep bidirectional transformers for language understanding. Annual Conference of the North American Chapter of the Association for Computational Linguistics. ↩︎
- Dzmitry Bahdanau, Kyunghyun Cho and Yoshua Bengio (2015). Neural Machine Translation by Jointly Learning to Align and Translate. International Conference on Learning Representations. ↩︎