jaxnet 0.1.2

Neural Nets for JAX

JAXnet

JAXnet is a neural net library for JAX. Different from popular neural net libraries, it is completely functional:

• No mutable weights in modules
• No global compute graph
• No global random key

This is an early version. Expect breaking changes! Install with

pip install jaxnet

If you want to run networks on GPU/TPU, first install the right version of jaxlib.

Overview

Defining networks looks similar to the TensorFlow2 / Keras functional API:

from jax import numpy as np, jit
from jax.random import PRNGKey
from jaxnet import *

net = Sequential([Conv(2, (3, 3)), relu, flatten, Dense(4), softmax])

To initialize parameter values for a network, call init_params on any module (with example inputs and a random key):

inputs = np.zeros((3, 5, 5, 1))
params = net.init_params(PRNGKey(0), inputs)

It initializes and returns all parameters, accessible via attributes:

print(params.layers[3].bias) # [0.00212132 0.01169001 0.00331698 0.00460713]

Invoke the network with:

output = net(params, inputs)

For acceleration use jit:

output = jit(net)(params, inputs)

See JAXnet in action in these demos: Mnist VAE and OCR with RNNs. Alternative design ideas are discussed here.

Defining modules

Modules are functions decorated with @parametrized, with parameters defined through default values:

def Dense(out_dim, kernel_init=glorot(), bias_init=randn()):
    @parametrized
    def dense(inputs,
              kernel=Param(lambda inputs: (inputs.shape[-1], out_dim), kernel_init),
              bias=Param(lambda _: (out_dim,), bias_init)):
        return np.dot(inputs, kernel) + bias

    return dense

Param specifies parameter shape and initialization function. @parametrized transforms this function to allow usage as above.

Nesting modules

Modules can be used in other modules through default arguments:

@parametrized
def net(inputs, layer1=Dense(10), layer2=Dense(20))
    inputs = layer1(inputs)
    return layer2(inputs)

Use many modules at once with collections:

def Sequential(layers):
    @parametrized
    def sequential(inputs, layers=layers):
        for module in layers:
            inputs = module(inputs)
        return inputs

    return sequential

Nested tuples/list/dicts of modules work. The same is true for Params.

Using parameter-free functions is seamless:

def relu(input):
    return np.maximum(input, 0)

layer = Sequential([Dense(10), relu])

Parameter sharing

Parameters can be shared by using module or parameter objects multiple times (not yet implemented):

shared_net=Sequential([layer, layer])

This is equivalent to (already implemented):

@parametrized
def shared_net(input, layer=layer):
    return layer(layer(input))

Parameter reuse

You can reuse parameters of submodules:

inputs = np.zeros((1, 2))

layer = Dense(5)
net1 = Sequential([layer, Dense(2)])
net2 = Sequential([layer, Dense(3)])

layer_params = layer.init_params(PRNGKey(0), inputs)
net1_params = net1.init_params(PRNGKey(0), inputs, reuse={layer: layer_params})
net2_params = net2.init_params(PRNGKey(1), inputs, reuse={layer: layer_params})

# Now net1_params.layers[0] equals net2_params.layers[0] equals layer_params

If all parameters are reused, you can use join_params instead of init_params:

inputs = np.zeros((1, 2))

net = Dense(5)
prediction = Sequential([net, softmax])

net_params = net.init_params(PRNGKey(0), inputs)
prediction_params = prediction.join_params({net: layer_params})

# net_params.layers[0] is now equal to net_params

output = jit(prediction)(prediction_params, inputs)

If you just want to call the network with these joined parameters, you can use the shorthand:

output = prediction.apply_joined({net: layer_params}, inputs, jit=True)

What about stax?

JAXnet is independent of stax. The main motivation over stax is to simplify nesting modules. Find details and porting instructions here.