#

Attention with Linear Biases (ALiBi)

This is an implementation of Attention with Linear Biases (ALiBi) from the paper Train Short, Test Long: Attention with Linear Biases Enables Input Length Extrapolation.

This replaces positional encodings with biases added to attention scores (attention logits, before the softmax). This is a relative scheme tested on autoregressive tasks, and the bias is higher for closeby tokens and lower for far-away tokens. The biases decrease linearly in the log scale (because it's before the softmax) and each head has a different slope.

Here's the attention formula for $i$ -th token,

a_{i} = soft m ax (q_{i} K^{⊤} + m \cdot [- (i - 1), \dots, 1, 0]) = soft m ax (q_{i} K^{⊤} + m \cdot [0, 1, \dots, (i - 1)])

where $q_{i} \in R^{d}$ is the query of the $i$ -th token, $K \in R^{i \times d}$ are the keys up to $i$ , and $d$ the number of features per head. Note that the above equality halts because $soft m ax$ is invariant to translations (you can add any constant to all elements without changing the result).

Here is the training code for a ALiBi model.

33import math
34from typing import Optional
35
36import torch
37from torch import nn
38
39from labml.logger import inspect
40from labml_nn.transformers.mha import MultiHeadAttention

#

Get head-specific slope $m$ for each head

n_heads is the number of heads in the attention layer $n$

The slope for first head is

$\frac{1}{2 ^{\frac{8}{n}}} = 2^{- \frac{8}{n}}$

The slopes for the rest of the heads are in a geometric series with a ratio same as above.

For instance when the number of heads is $8$ the slopes are $\frac{1}{2 ^{1}}, \frac{1}{2 ^{2}}, \dots, \frac{1}{2 ^{8}}$

43def get_slopes(n_heads: int):

#

Get the closest power of 2 to n_heads . If n_heads is not a power of 2, then we first calculate slopes to the closest (smaller) power of 2, and then add the remaining slopes.

62    n = 2 ** math.floor(math.log2(n_heads))

#

$2^{- \frac{8}{n}}$

64    m_0 = 2.0 ** (-8.0 / n)

#

$2^{- 1 \frac{8}{n}}, 2^{- 2 \frac{8}{n}}, 2^{- 3 \frac{8}{n}}, \dots$

66    m = torch.pow(m_0, torch.arange(1, 1 + n))

#

If n_heads is not a power of 2, then we add the remaining slopes. We calculate the remaining slopes for $n * 2$ (avoiding slopes added previously). And pick the slopes upto n_heads .

71    if n < n_heads:

#

$2^{- \frac{8}{2 n}}$

73        m_hat_0 = 2.0 ** (-4.0 / n)

#

$2^{- 1 \frac{8}{2 n}}, 2^{- 3 \frac{8}{2 n}}, 2^{- 5 \frac{8}{2 n}}, \dots$ Note that we take steps by $2$ to avoid slopes added previously.

76        m_hat = torch.pow(m_hat_0, torch.arange(1, 1 + 2 * (n_heads - n), 2))

#

Concatenate the slopes with the remaining slopes.

78        m = torch.cat([m, m_hat])
79
80    return m

#

Calculate the attention biases matrix

n_heads is the number of heads in the attention layer
mask is the attention mask of shape [seq_len_q, seq_len_k]

This returns a matrix of shape [seq_len_q, seq_len_k, n_heads, ] with ALiBi attention biases.

83@torch.no_grad()
84def get_alibi_biases(n_heads: int, mask: torch.Tensor):

#

Get slopes $m$ for each head

95    m = get_slopes(n_heads).to(mask.device)

#

Calculate distances $[0, 1, \dots, N]$ Here we calculate the distances using the mask.

Since it's causal mask we can just use $[0, 1, \dots, N]$ too. distance = torch.arange(mask.shape[1], dtype=torch.long, device=mask.device)[None, :]

102    distance = mask.cumsum(dim=-1)

#

Multiply them pair-wise to get the AliBi bias matrix

105    return distance[:, :, None] * m[None, None, :]

#

Attention with Linear Biases (ALiBi)

We override Multi-Head Attention.

108class AlibiMultiHeadAttention(MultiHeadAttention):

#

115    def __init__(self, heads: int, d_model: int, dropout_prob: float = 0.1):
116        super().__init__(heads, d_model, dropout_prob)

#

To cache AliBi the biases

119        self.alibi_biases = None

#

query , key and value are the tensors that store collection of query, key and value vectors. They have shape [seq_len, batch_size, d_model] .

mask has shape [seq_len, seq_len, batch_size] and mask[i, j, b] indicates whether for batch b , query at position i has access to key-value at position j .

121    def forward(self, *,
122                query: torch.Tensor,
123                key: torch.Tensor,
124                value: torch.Tensor,
125                mask: Optional[torch.Tensor] = None):

#

ALiBi only works with causal masks.

137        assert mask is not None
138        assert mask.shape[0] == mask.shape[1] and mask.shape[2] == 1

#

query , key and value have shape [seq_len, batch_size, d_model]

141        seq_len, batch_size, _ = query.shape

#

Add head dimension to mask and check its shape.

144        mask = self.prepare_mask(mask, query.shape, key.shape)

#

Prepare query , key and value for attention computation. These will then have shape [seq_len, batch_size, heads, d_k] .

148        query = self.query(query)
149        key = self.key(key)
150        value = self.value(value)

#

Compute attention scores $Q K^{⊤}$ . This gives a tensor of shape [seq_len, seq_len, batch_size, heads] .

154        scores = self.get_scores(query, key)

#

Scale scores $\frac{Q K ^{⊤}}{d _{k}}$

157        scores *= self.scale

#

Create AliBi biases if it's not cached

160        if self.alibi_biases is None or self.alibi_biases.shape[1] < seq_len:

#

mask has shape seq_len, seq_len, 1, 1

162            self.alibi_biases = get_alibi_biases(scores.shape[-1], mask[:, :, 0, 0])

#

Add AliBi biases to attention scores. ALiBi biases has shape [seq_len, seq_len, n_heads] and scores has shape [seq_len, seq_len, batch_size, n_heads]

167        scores += self.alibi_biases[:seq_len, :seq_len, None, :]

#

Apply mask

170        scores = scores.masked_fill(mask == 0, float('-inf'))

#

$so f t m a x$ attention along the key sequence dimension $se q so f t m a x (\frac{Q K ^{⊤}}{d _{k}})$

174        attn = self.softmax(scores)

#

Apply dropout

177        attn = self.dropout(attn)

#

Multiply by values $se q so f t m a x (\frac{Q K ^{⊤}}{d _{k}}) V$

181        x = torch.einsum("ijbh,jbhd->ibhd", attn, value)

#

Concatenate multiple heads

184        x = x.reshape(seq_len, batch_size, -1)

#

Output layer

187        return self.output(x)

#

Simple test function to see the slopes.

190def _test_alibi():

#

194    inspect(get_slopes(12).tolist(), _n=-1)
195    from labml_nn.transformers.utils import subsequent_mask
196
197    mask = subsequent_mask(8)[:, :, 0]
198    inspect(mask)
199
200    inspect(get_alibi_biases(12, mask)[:, :, 3], _n=-1)

#

204if __name__ == '__main__':
205    _test_alibi()

Attention with Linear Biases (ALiBi)

Get head-specific slope m for each head

Calculate the attention biases matrix

Attention with Linear Biases (ALiBi)

Get head-specific slope $m$ for each head