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# Copyright 2023 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Transformer model helper methods."""
import math
import numpy as np
import tensorflow as tf, tf_keras
# Very low numbers to represent -infinity. We do not actually use -Inf, since we
# want to be able to multiply these values by zero to get zero. (-Inf * 0 = NaN)
_NEG_INF_FP32 = -1e9
_NEG_INF_FP16 = np.finfo(np.float16).min
def get_position_encoding(length,
hidden_size,
min_timescale=1.0,
max_timescale=1.0e4):
"""Return positional encoding.
Calculates the position encoding as a mix of sine and cosine functions with
geometrically increasing wavelengths.
Defined and formulized in Attention is All You Need, section 3.5.
Args:
length: Sequence length.
hidden_size: Size of the
min_timescale: Minimum scale that will be applied at each position
max_timescale: Maximum scale that will be applied at each position
Returns:
Tensor with shape [length, hidden_size]
"""
# We compute the positional encoding in float32 even if the model uses
# float16, as many of the ops used, like log and exp, are numerically unstable
# in float16.
position = tf.cast(tf.range(length), tf.float32)
num_timescales = hidden_size // 2
log_timescale_increment = (
math.log(float(max_timescale) / float(min_timescale)) /
(tf.cast(num_timescales, tf.float32) - 1))
inv_timescales = min_timescale * tf.exp(
tf.cast(tf.range(num_timescales), tf.float32) * -log_timescale_increment)
scaled_time = tf.expand_dims(position, 1) * tf.expand_dims(inv_timescales, 0)
signal = tf.concat([tf.sin(scaled_time), tf.cos(scaled_time)], axis=1)
return signal
def get_decoder_self_attention_bias(length, dtype=tf.float32):
"""Calculate bias for decoder that maintains model's autoregressive property.
Creates a tensor that masks out locations that correspond to illegal
connections, so prediction at position i cannot draw information from future
positions.
Args:
length: int length of sequences in batch.
dtype: The dtype of the return value.
Returns:
float tensor of shape [1, 1, length, length]
"""
neg_inf = _NEG_INF_FP16 if dtype == tf.float16 else _NEG_INF_FP32
with tf.name_scope("decoder_self_attention_bias"):
valid_locs = tf.linalg.band_part(
tf.ones([length, length], dtype=dtype), -1, 0)
valid_locs = tf.reshape(valid_locs, [1, 1, length, length])
decoder_bias = neg_inf * (1.0 - valid_locs)
return decoder_bias
def get_padding(x, padding_value=0, dtype=tf.float32):
"""Return float tensor representing the padding values in x.
Args:
x: int tensor with any shape
padding_value: int which represents padded values in input
dtype: The dtype of the return value.
Returns:
float tensor with same shape as x containing values 0 or 1.
0 -> non-padding, 1 -> padding
"""
with tf.name_scope("padding"):
return tf.cast(tf.equal(x, padding_value), dtype)
def get_padding_bias(x, padding_value=0, dtype=tf.float32):
"""Calculate bias tensor from padding values in tensor.
Bias tensor that is added to the pre-softmax multi-headed attention logits,
which has shape [batch_size, num_heads, length, length]. The tensor is zero at
non-padding locations, and -1e9 (negative infinity) at padding locations.
Args:
x: int tensor with shape [batch_size, length]
padding_value: int which represents padded values in input
dtype: The dtype of the return value
Returns:
Attention bias tensor of shape [batch_size, 1, 1, length].
"""
with tf.name_scope("attention_bias"):
padding = get_padding(x, padding_value, dtype)
attention_bias = padding * _NEG_INF_FP32
attention_bias = tf.expand_dims(
tf.expand_dims(attention_bias, axis=1), axis=1)
return attention_bias
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