tf-seq2seq-losses

Tensorflow implementations for Connectionist Temporal Classification (CTC) loss functions that are fast and support second-order derivatives.

Installation

$ pip install tf-seq2seq-losses

Why Use This Package?

1. Faster Performance

Official CTC loss implementation, tf.nn.ctc_loss, is significantly slower. Our implementation is approximately 30 times faster, as shown by the benchmark results:

Tested on a single GPU: GeForce GTX 970, Driver Version: 460.91.03, CUDA Version: 11.2. For the experimental setup, see benchmark.py To reproduce this benchmark, run the following command from the project root directory (install pytest and pandas if needed):

$ pytest -o log_cli=true --log-level=INFO tests/benchmark.py

Here, classic_ctc_loss is the standard version of CTC loss with token collapsing, e.g., a_bb_ccc_c -> abcc. The simple_ctc_loss is a simplified version that removes blanks trivially, e.g., a_bb_ccc_c -> abbcccc.

2. Supports Second-Order Derivatives

This implementation supports second-order derivatives without using TensorFlow's autogradient. Instead, it uses a custom approach similar to the one described here with a complexity of $O(l^4)$, where $l$ is the sequence length. The gradient complexity is $O(l^2)$.

Example usage:

import tensorflow as tf
from tf_seq2seq_losses import classic_ctc_loss 

batch_size = 2
num_tokens = 3
logit_length = 5
labels = tf.constant([[1, 2, 2, 1], [1, 2, 1, 0]], dtype=tf.int32)
label_length = tf.constant([4, 3], dtype=tf.int32)
logits = tf.zeros(shape=[batch_size, logit_length, num_tokens], dtype=tf.float32)
logit_length = tf.constant([5, 4], dtype=tf.int32)

with tf.GradientTape(persistent=True) as tape1: 
    tape1.watch([logits])
    with tf.GradientTape() as tape2:
        tape2.watch([logits])
        loss = tf.reduce_sum(classic_ctc_loss(
            labels=labels,
            logits=logits,
            label_length=label_length,
            logit_length=logit_length,
            blank_index=0,
        ))
    gradient = tape2.gradient(loss, sources=logits)
hessian = tape1.batch_jacobian(gradient, source=logits, experimental_use_pfor=False)
# shape = [2, 5, 3, 5, 3]

3. Numerical Stability

1. The proposed implementation is more numerically stable, producing reasonable outputs even for logits of order 1e+10 and -tf.inf.
2. If the logit length is too short to predict the label output, the loss is tf.inf for that sample, unlike tf.nn.ctc_loss, which might output 707.13184.

4. Pure Python Implementation

This is a pure Python/TensorFlow implementation, eliminating the need to build or compile any C++/CUDA components.

Usage

The interface is identical to tensorflow.nn.ctc_loss with logits_time_major=False.

Example:

import tensorflow as tf
from tf_seq2seq_losses import classic_ctc_loss

batch_size = 1
num_tokens = 3 # = 2 tokens + 1 blank token
logit_length = 5
loss = classic_ctc_loss(
    labels=tf.constant([[1, 2, 2, 1]], dtype=tf.int32),
    logits=tf.zeros(shape=[batch_size, logit_length, num_tokens], dtype=tf.float32),
    label_length=tf.constant([4], dtype=tf.int32),
    logit_length=tf.constant([logit_length], dtype=tf.int32),
    blank_index=0,
)

Under the Hood

The implementation uses TensorFlow operations such as tf.while_loop and tf.TensorArray. The main computational bottleneck is the iteration over the logit length to calculate α and β (as described in the original CTC paper). The expected gradient GPU calculation time is linear with respect to the logit length.

Known Issues

1. Warning:

AutoGraph could not transform <function classic_ctc_loss at ...> and will run it as-is. Please report this to the TensorFlow team. When filing the bug, set the verbosity to 10 (on Linux, export AUTOGRAPH_VERBOSITY=10) and attach the full output.

Observed with TensorFlow version 2.4.1. This warning does not affect performance and is caused by the use of Union in type annotations.

2. UnimplementedError:

Using tf.jacobian and tf.batch_jacobian for the second derivative of classic_ctc_loss with experimental_use_pfor=False in tf.GradientTape may cause an unexpected UnimplementedError in TensorFlow version 2.4.1 or later. This can be avoided by setting experimental_use_pfor=True or by using ClassicCtcLossData.hessian directly without tf.GradientTape.

Feel free to reach out if you have any questions or need further clarification.