Mini-libraries

JAX provides some small, experimental libraries for machine learning. These libraries are in part about providing tools and in part about serving as examples for how to build such libraries using JAX. Each one is only <300 source lines of code, so take a look inside and adapt them as you need!

👉 Note: each mini-library is meant to be an inspiration, but not a prescription.

To serve that purpose, it is best to keep their code samples minimal; so we generally will not merge PRs adding new features. Instead, please send your lovely pull requests and design ideas to more fully-featured libraries like Haiku, Flax, or Trax.

Neural-net building with Stax

Stax is a functional neural network building library. The basic idea is that a single layer or an entire network can be modeled as an (init_fun, apply_fun) pair. The init_fun is used to initialize network parameters and the apply_fun takes parameters and inputs to produce outputs. There are constructor functions for common basic pairs, like Conv and Relu, and these pairs can be composed in series using stax.serial or in parallel using stax.parallel.

Here's an example:

import jax.numpy as jnp
from jax import random
from jax.example_libraries import stax
from jax.example_libraries.stax import (
    Conv, Dense, MaxPool, Relu, Flatten, LogSoftmax)

# Use stax to set up network initialization and evaluation functions
net_init, net_apply = stax.serial(
    Conv(32, (3, 3), padding='SAME'), Relu,
    Conv(64, (3, 3), padding='SAME'), Relu,
    MaxPool((2, 2)), Flatten,
    Dense(128), Relu,
    Dense(10), LogSoftmax,
)

# Initialize parameters, not committing to a batch shape
rng = random.key(0)
in_shape = (-1, 28, 28, 1)
out_shape, net_params = net_init(rng, in_shape)

# Apply network to dummy inputs
inputs = jnp.zeros((128, 28, 28, 1))
predictions = net_apply(net_params, inputs)

First-order optimization

The file optimizers.py contains a minimal optimization library focused on stochastic first-order optimizers. Every optimizer is modeled as an (init_fun, update_fun, get_params) triple of functions. The init_fun is used to initialize the optimizer state, which could include things like momentum variables, and the update_fun accepts a gradient and an optimizer state to produce a new optimizer state. The get_params function extracts the current iterate (i.e. the current parameters) from the optimizer state. The parameters being optimized can be ndarrays or arbitrarily-nested list/tuple/dict structures, so you can store your parameters however you'd like.

Here's an example, using jit to compile the whole update end-to-end:

from jax.example_libraries import optimizers
from jax import jit, grad

# Define a simple squared-error loss
def loss(params, batch):
  inputs, targets = batch
  predictions = net_apply(params, inputs)
  return jnp.sum((predictions - targets)**2)

# Use optimizers to set optimizer initialization and update functions
opt_init, opt_update, get_params = optimizers.momentum(step_size=1e-3, mass=0.9)

# Define a compiled update step
@jit
def step(i, opt_state, batch):
  params = get_params(opt_state)
  g = grad(loss)(params, batch)
  return opt_update(i, g, opt_state)

# Dummy input data stream
data_generator = ((jnp.zeros((128, 28, 28, 1)), jnp.zeros((128, 10)))
                  for _ in range(10))

# Optimize parameters in a loop
opt_state = opt_init(net_params)
for i in range(10):
  opt_state = step(i, opt_state, next(data_generator))
net_params = get_params(opt_state)

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