Sinusoidal positional encoding · Ingénierie IA

Attention does not see the order of the tokens. The original transformer adds a fixed table of sines and cosines to the token embeddings. Each row of the table is the code of one position, and each pair of columns turns at its own frequency.

Task

Write two functions.

sinusoidal_pe(max_len, d_model, base=10000.0) returns the table of shape (max_len, d_model).
add_positional_encoding(x, base=10000.0) returns x plus the table for its sequence length.

For position p = 0, 1, \dots, \text{max\_len} - 1 and pair i = 0, 1, \dots, \lfloor d/2 \rfloor - 1, with d = d_model:

PE[p, 2i] = \sin\left(p / \text{base}^{2i/d}\right), \qquad PE[p, 2i+1] = \cos\left(p / \text{base}^{2i/d}\right)

The sin and the cos of one pair are in adjacent columns. When d_model is odd, the last column has no pair. It stays 0. The exponent still divides by d_model.

add_positional_encoding adds row t of the table to the vector of token t. The tokens are on the second to last axis of x. Every leading axis gets the same table.

Input and output

Name	Type	Shape	Meaning
`max_len`	int		the number of positions
`d_model`	int		the number of columns
`base`	float		the base of the frequencies
`x`	float array	`(..., T, d_model)`	the embeddings of `T` tokens, with zero or more leading batch axes

sinusoidal_pe returns a float array of shape (max_len, d_model).
add_positional_encoding returns a new float array with the shape of x.

Constraints

1 <= max_len <= 8192 and 1 <= d_model <= 1024.
Positions start at 0.
Do not change the input arrays.
Use numpy only.

Examples

python

sinusoidal_pe(3, 4)

text

[[ 0.          1.          0.          1.        ]
 [ 0.84147098  0.54030231  0.00999983  0.99995   ]
 [ 0.90929743 -0.41614684  0.01999867  0.99980001]]

python

sinusoidal_pe(2, 5)

text

[[0.         1.         0.         1.         0.        ]
 [0.84147098 0.54030231 0.02511622 0.99968454 0.        ]]

python

add_positional_encoding(np.ones((2, 2)))

text

[[1.         2.        ]
 [1.84147098 1.54030231]]