Haiku: Sonnet for JAX

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[!IMPORTANT]
📣 As of July 2023 Google DeepMind recommends that new projects adopt
Flax instead of Haiku. Flax is a neural network library originally
developed by Google Brain and now by Google DeepMind. 📣

At the time of writing Flax has superset of the features available in Haiku,
a larger and
more active development team and
more adoption with users outside of Alphabet. Flax has
more extensive documentation,
examples
and an active community creating end
to end examples.

Haiku will remain best-effort supported, however the project will enter
maintenance mode, meaning
that development efforts will be focussed on bug fixes and compatibility with
new releases of JAX.

New releases will be made to keep Haiku working with newer versions of Python
and JAX, however we will not be adding (or accepting PRs for) new features.

We have significant usage of Haiku internally at Google DeepMind and
currently plan to support Haiku in this mode indefinitely.

What is Haiku?

Haiku is a tool

For building neural networks

Think: "Sonnet for JAX"

Haiku is a simple neural network library for JAX developed by some of the
authors of Sonnet, a neural network library for TensorFlow.

Documentation on Haiku can be found at https://dm-haiku.readthedocs.io/.

Disambiguation: if you are looking for Haiku the operating system then
please see https://haiku-os.org/.

Overview

JAX is a numerical computing library that combines NumPy, automatic
differentiation, and first-class GPU/TPU support.

Haiku is a simple neural network library for JAX that enables users to use
familiar object-oriented programming models while allowing full access to
JAX's pure function transformations.

Haiku provides two core tools: a module abstraction, hk.Module, and a simple
function transformation, hk.transform.

hk.Modules are Python objects that hold references to their own parameters,
other modules, and methods that apply functions on user inputs.

hk.transform turns functions that use these object-oriented, functionally
"impure" modules into pure functions that can be used with jax.jit,
jax.grad, jax.pmap, etc.

Why Haiku?

There are a number of neural network libraries for JAX. Why should you choose
Haiku?

Haiku has been tested by researchers at DeepMind at scale.

DeepMind has reproduced a number of experiments in Haiku and JAX with relative
ease. These include large-scale results in image and language processing,
generative models, and reinforcement learning.

Haiku is a library, not a framework.

Haiku is designed to make specific things simpler: managing model parameters
and other model state.
Haiku can be expected to compose with other libraries and work well with the
rest of JAX.
Haiku otherwise is designed to get out of your way - it does not define custom
optimizers, checkpointing formats, or replication APIs.

Haiku does not reinvent the wheel.

Haiku builds on the programming model and APIs of Sonnet, a neural network
library with near universal adoption at DeepMind. It preserves Sonnet's
Module-based programming model for state management while retaining access
to JAX's function transformations.
Haiku APIs and abstractions are as close as reasonable to Sonnet. Many users
have found Sonnet to be a productive programming model in TensorFlow; Haiku
enables the same experience in JAX.

Transitioning to Haiku is easy.

By design, transitioning from TensorFlow and Sonnet to JAX and Haiku is easy.
Outside of new features (e.g. hk.transform), Haiku aims to match the API of
Sonnet 2. Modules, methods, argument names, defaults, and initialization
schemes should match.

Haiku makes other aspects of JAX simpler.

Haiku offers a trivial model for working with random numbers. Within a
transformed function, hk.next_rng_key() returns a unique rng key.
These unique keys are deterministically derived from an initial random key
passed into the top-level transformed function, and are thus safe to use with
JAX program transformations.

Quickstart

Let's take a look at an example neural network, loss function, and training
loop. (For more examples, see our
examples directory.
The
MNIST example
is a good place to start.)

import haiku as hk
import jax.numpy as jnp

def softmax_cross_entropy(logits, labels):
  one_hot = jax.nn.one_hot(labels, logits.shape[-1])
  return -jnp.sum(jax.nn.log_softmax(logits) * one_hot, axis=-1)

def loss_fn(images, labels):
  mlp = hk.Sequential([
      hk.Linear(300), jax.nn.relu,
      hk.Linear(100), jax.nn.relu,
      hk.Linear(10),
  ])
  logits = mlp(images)
  return jnp.mean(softmax_cross_entropy(logits, labels))

loss_fn_t = hk.transform(loss_fn)
loss_fn_t = hk.without_apply_rng(loss_fn_t)

rng = jax.random.PRNGKey(42)
dummy_images, dummy_labels = next(input_dataset)
params = loss_fn_t.init(rng, dummy_images, dummy_labels)

def update_rule(param, update):
  return param - 0.01 * update

for images, labels in input_dataset:
  grads = jax.grad(loss_fn_t.apply)(params, images, labels)
  params = jax.tree.map(update_rule, params, grads)

The core of Haiku is hk.transform. The transform function allows you to
write neural network functions that rely on parameters (here the weights of the
Linear layers) without requiring you to explicitly write the boilerplate
for initialising those parameters. transform does this by transforming the
function into a pair of functions that are pure (as required by JAX) init
and apply.

`init`

The init function, with signature params = init(rng, ...) (where ... are
the arguments to the untransformed function), allows you to collect the
initial value of any parameters in the network. Haiku does this by running your
function, keeping track of any parameters requested through hk.get_parameter
(called by e.g. hk.Linear) and returning them to you.

The params object returned is a nested data structure of all the
parameters in your network, designed for you to inspect and manipulate.
Concretely, it is a mapping of module name to module parameters, where a module
parameter is a mapping of parameter name to parameter value. For example:

{'linear': {'b': ndarray(..., shape=(300,), dtype=float32),
            'w': ndarray(..., shape=(28, 300), dtype=float32)},
 'linear_1': {'b': ndarray(..., shape=(100,), dtype=float32),
              'w': ndarray(..., shape=(1000, 100), dtype=float32)},
 'linear_2': {'b': ndarray(..., shape=(10,), dtype=float32),
              'w': ndarray(..., shape=(100, 10), dtype=float32)}}

`apply`

The apply function, with signature result = apply(params, rng, ...), allows
you to inject parameter values into your function. Whenever
hk.get_parameter is called, the value returned will come from the params you
provide as input to apply:

loss = loss_fn_t.apply(params, rng, images, labels)

Note that since the actual computation performed by our loss function doesn't
rely on random numbers, passing in a random number generator is unnecessary, so
we could also pass in None for the rng argument. (Note that if your
computation does use random numbers, passing in None for rng will cause
an error to be raised.) In our example above, we ask Haiku to do this for us
automatically with:

loss_fn_t = hk.without_apply_rng(loss_fn_t)

Since apply is a pure function we can pass it to jax.grad (or any of JAX's
other transforms):

grads = jax.grad(loss_fn_t.apply)(params, images, labels)

Training

The training loop in this example is very simple. One detail to note is the use
of jax.tree.map to apply the sgd function across all matching entries in
params and grads. The result has the same structure as the previous params
and can again be used with apply.

Installation

Haiku is written in pure Python, but depends on C++ code via JAX.

Because JAX installation is different depending on your CUDA version, Haiku does
not list JAX as a dependency in requirements.txt.

First, follow these instructions
to install JAX with the relevant accelerator support.

Then, install Haiku using pip:

$ pip install git+https://github.com/deepmind/dm-haiku

Alternatively, you can install via PyPI:

$ pip install -U dm-haiku

Our examples rely on additional libraries (e.g. bsuite). You can install the full set of additional requirements using pip:

$ pip install -r examples/requirements.txt

User manual

Writing your own modules

In Haiku, all modules are a subclass of hk.Module. You can implement any
method you like (nothing is special-cased), but typically modules implement
__init__ and __call__.

Let's work through implementing a linear layer:

class MyLinear(hk.Module):

  def __init__(self, output_size, name=None):
    super().__init__(name=name)
    self.output_size = output_size

  def __call__(self, x):
    j, k = x.shape[-1], self.output_size
    w_init = hk.initializers.TruncatedNormal(1. / np.sqrt(j))
    w = hk.get_parameter("w", shape=[j, k], dtype=x.dtype, init=w_init)
    b = hk.get_parameter("b", shape=[k], dtype=x.dtype, init=jnp.zeros)
    return jnp.dot(x, w) + b

All modules have a name. When no name argument is passed to the module, its
name is inferred from the name of the Python class (for example MyLinear
becomes my_linear). Modules can have named parameters that are accessed
using hk.get_parameter(param_name, ...). We use this API (rather than just
using object properties) so that we can convert your code into a pure function
using hk.transform.

When using modules you need to define functions and transform them into a pair
of pure functions using hk.transform. See our quickstart for
more details about the functions returned from transform:

def forward_fn(x):
  model = MyLinear(10)
  return model(x)

# Turn `forward_fn` into an object with `init` and `apply` methods. By default,
# the `apply` will require an rng (which can be None), to be used with
# `hk.next_rng_key`.
forward = hk.transform(forward_fn)

x = jnp.ones([1, 1])

# When we run `forward.init`, Haiku will run `forward_fn(x)` and collect initial
# parameter values. Haiku requires you pass a RNG key to `init`, since parameters
# are typically initialized randomly:
key = hk.PRNGSequence(42)
params = forward.init(next(key), x)

# When we run `forward.apply`, Haiku will run `forward_fn(x)` and inject parameter
# values from the `params` that are passed as the first argument.  Note that
# models transformed using `hk.transform(f)` must be called with an additional
# `rng` argument: `forward.apply(params, rng, x)`. Use
# `hk.without_apply_rng(hk.transform(f))` if this is undesirable.
y = forward.apply(params, None, x)

Working with stochastic models

Some models may require random sampling as part of the computation.
For example, in variational autoencoders with the reparametrization trick,
a random sample from the standard normal distribution is needed. For dropout we
need a random mask to drop units from the input. The main hurdle in making this
work with JAX is in management of PRNG keys.

In Haiku we provide a simple API for maintaining a PRNG key sequence associated
with modules: hk.next_rng_key() (or next_rng_keys() for multiple keys):

class MyDropout(hk.Module):

  def __init__(self, rate=0.5, name=None):
    super().__init__(name=name)
    self.rate = rate

  def __call__(self, x):
    key = hk.next_rng_key()
    p = jax.random.bernoulli(key, 1.0 - self.rate, shape=x.shape)
    return x * p / (1.0 - self.rate)

forward = hk.transform(lambda x: MyDropout()(x))

key1, key2 = jax.random.split(jax.random.PRNGKey(42), 2)
params = forward.init(key1, x)
prediction = forward.apply(params, key2, x)

For a more complete look at working with stochastic models, please see our
VAE example.

Note: hk.next_rng_key() is not functionally pure which means you should
avoid using it alongside JAX transformations which are inside hk.transform.
For more information and possible workarounds, please consult the docs on
Haiku transforms
and available
wrappers for JAX transforms inside Haiku networks.

Working with non-trainable state

Some models may want to maintain some internal, mutable state. For example, in
batch normalization a moving average of values encountered during training is
maintained.

In Haiku we provide a simple API for maintaining mutable state that is
associated with modules: hk.set_state and hk.get_state. When using these
functions you need to transform your function using hk.transform_with_state
since the signature of the returned pair of functions is different:

def forward(x, is_training):
  net = hk.nets.ResNet50(1000)
  return net(x, is_training)

forward = hk.transform_with_state(forward)

# The `init` function now returns parameters **and** state. State contains
# anything that was created using `hk.set_state`. The structure is the same as
# params (e.g. it is a per-module mapping of named values).
params, state = forward.init(rng, x, is_training=True)

# The apply function now takes both params **and** state. Additionally it will
# return updated values for state. In the resnet example this will be the
# updated values for moving averages used in the batch norm layers.
logits, state = forward.apply(params, state, rng, x, is_training=True)

If you forget to use hk.transform_with_state don't worry, we will print a
clear error pointing you to hk.transform_with_state rather than silently
dropping your state.

Distributed training with `jax.pmap`

The pure functions returned from hk.transform (or hk.transform_with_state)
are fully compatible with jax.pmap. For more details on SPMD programming with
jax.pmap,
look here.

One common use of jax.pmap with Haiku is for data-parallel training on many
accelerators, potentially across multiple hosts. With Haiku, that might look
like this:

def loss_fn(inputs, labels):
  logits = hk.nets.MLP([8, 4, 2])(x)
  return jnp.mean(softmax_cross_entropy(logits, labels))

loss_fn_t = hk.transform(loss_fn)
loss_fn_t = hk.without_apply_rng(loss_fn_t)

# Initialize the model on a single device.
rng = jax.random.PRNGKey(428)
sample_image, sample_label = next(input_dataset)
params = loss_fn_t.init(rng, sample_image, sample_label)

# Replicate params onto all devices.
num_devices = jax.local_device_count()
params = jax.tree.map(lambda x: np.stack([x] * num_devices), params)

def make_superbatch():
  """Constructs a superbatch, i.e. one batch of data per device."""
  # Get N batches, then split into list-of-images and list-of-labels.
  superbatch = [next(input_dataset) for _ in range(num_devices)]
  superbatch_images, superbatch_labels = zip(*superbatch)
  # Stack the superbatches to be one array with a leading dimension, rather than
  # a python list. This is what `jax.pmap` expects as input.
  superbatch_images = np.stack(superbatch_images)
  superbatch_labels = np.stack(superbatch_labels)
  return superbatch_images, superbatch_labels

def update(params, inputs, labels, axis_name='i'):
  """Updates params based on performance on inputs and labels."""
  grads = jax.grad(loss_fn_t.apply)(params, inputs, labels)
  # Take the mean of the gradients across all data-parallel replicas.
  grads = jax.lax.pmean(grads, axis_name)
  # Update parameters using SGD or Adam or ...
  new_params = my_update_rule(params, grads)
  return new_params

# Run several training updates.
for _ in range(10):
  superbatch_images, superbatch_labels = make_superbatch()
  params = jax.pmap(update, axis_name='i')(params, superbatch_images,
                                           superbatch_labels)

For a more complete look at distributed Haiku training, take a look at our
ResNet-50 on ImageNet example.

Citing Haiku

To cite this repository:

@software{haiku2020github,
  author = {Tom Hennigan and Trevor Cai and Tamara Norman and Lena Martens and Igor Babuschkin},
  title = {{H}aiku: {S}onnet for {JAX}},
  url = {http://github.com/deepmind/dm-haiku},
  version = {0.0.14},
  year = {2020},
}

In this bibtex entry, the version number is intended to be from
haiku/__init__.py,
and the year corresponds to the project's open-source release.

Dependency	Optional
absl-py	false
jmp	false
numpy	false
tabulate	false
flax	true
jax	true
jaxlib	true

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