MLJFlux
An interface to the Flux deep learning models for the MLJ machine learning framework
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MLJFlux makes it possible to apply the machine learning metaalgorithms provided by MLJ  such as outofsample performance evaluation and hyperparameter optimization  to some classes of supervised deep learning models. It does this by providing an interface to the Flux framework.
The guiding vision of this package is to make evaluating and optimizing basic Flux models more convenient to users already familiar with the MLJ workflow. This goal will likely place restrictions of the class of Flux models that can used, at least in the medium term. For example, online learning, reenforcement learning, and adversarial networks are currently out of scope.
Basic idea
Each MLJFlux model has a builder hyperparameter, an object encoding instructions for creating a neural network given the data that the model eventually sees (e.g., the number of classes in a classification problem). While each MLJ model has a simple default builder, users will generally need to define their own builders to get good results, and this will require familiarity with the Flux API for defining a neural network chain.
In the future MLJFlux may provide a larger assortment of canned builders. Pull requests introducing new ones are most welcome.
Installation
using Pkg
Pkg.activate("my_environment", shared=true)
Pkg.add("MLJFlux")
Pkg.add("MLJ")
Pkg.add("RDatasets") # for the demo below
Example
Following is an introductory example using a default builder and no standardization of input features (notebook/script).
For an example implementing early stopping and snapshots, using MLJ's
IteratedModel
wrapper,
see the MNIST dataset
example.
Loading some data and instantiating a model
using MLJ
import RDatasets
iris = RDatasets.dataset("datasets", "iris");
y, X = unpack(iris, ==(:Species), colname > true, rng=123);
NeuralNetworkClassifier = @load NeuralNetworkClassifier
julia> clf = NeuralNetworkClassifier()
NeuralNetworkClassifier(
builder = Short(
n_hidden = 0,
dropout = 0.5,
σ = NNlib.σ),
finaliser = NNlib.softmax,
optimiser = ADAM(0.001, (0.9, 0.999), IdDict{Any,Any}()),
loss = Flux.crossentropy,
epochs = 10,
batch_size = 1,
lambda = 0.0,
alpha = 0.0,
optimiser_changes_trigger_retraining = false) @ 1…60
Incremental training
import Random.seed!; seed!(123)
mach = machine(clf, X, y)
fit!(mach)
julia> training_loss = cross_entropy(predict(mach, X), y) > mean
0.89526004f0
# Increasing learning rate and adding iterations:
clf.optimiser.eta = clf.optimiser.eta * 2
clf.epochs = clf.epochs + 5
julia> fit!(mach, verbosity=2)
[ Info: Updating Machine{NeuralNetworkClassifier{Short,…}} @240.
[ Info: Loss is 0.853
[ Info: Loss is 0.8207
[ Info: Loss is 0.8072
[ Info: Loss is 0.752
[ Info: Loss is 0.7077
Machine{NeuralNetworkClassifier{Short,…}} @ 1…42
julia> training_loss = cross_entropy(predict(mach, X), y) > mean
0.7076618f0
Accessing the Flux chain (model)
julia> fitted_params(mach).chain
Chain(Chain(Dense(4, 3, σ), Flux.Dropout{Float64}(0.5, false), Dense(3, 3)), softmax)
Evolution of outofsample performance
r = range(clf, :epochs, lower=1, upper=200, scale=:log10)
curve = learning_curve(clf, X, y,
range=r,
resampling=Holdout(fraction_train=0.7),
measure=cross_entropy)
using Plots
plot(curve.parameter_values,
curve.measurements,
xlab=curve.parameter_name,
xscale=curve.parameter_scale,
ylab = "Cross Entropy")
Models
In MLJ a model is a mutable struct storing hyperparameters for some learning algorithm indicated by the model name, and that's all. In particular, an MLJ model does not store learned parameters.
Warning: In Flux the term "model" has another meaning. However, as all
Flux "models" used in MLJFLux are Flux.Chain
objects, we call them
chains, and restrict use of "model" to models in the MLJ sense.
MLJFlux provides four model types, for use with input features X
and
targets y
of the scientific
type
indicated in the table below. The parameters n_in
and n_out
refer to information passed to the builder, as described under
Defining a new builder below.
model type  prediction type  scitype(X) <: _ 
scitype(y) <: _ 

NeuralNetworkRegressor 
Deterministic 
Table(Continuous) with n_in columns 
AbstractVector{<:Continuous) (n_out = 1 ) 
MultitargetNeuralNetworkRegressor 
Deterministic 
Table(Continuous) with n_in columns 
<: Table(Continuous) with n_out columns 
NeuralNetworkClassifier 
Probabilistic 
<:Table(Continuous) with n_in columns 
AbstractVector{<:Finite} with n_out classes 
ImageClassifier 
Probabilistic 
AbstractVector(<:Image{W,H}) with n_in = (W, H) 
AbstractVector{<:Finite} with n_out classes 
Table 1. Input and output types for MLJFlux models
Nontabular input
Any AbstractMatrix{<:AbstractFloat}
object Xmat
can be forced to
have scitype Table(Continuous)
by replacing it with X = MLJ.table(Xmat)
. Furthermore, this wrapping, and subsequent
unwrapping under the hood, will compile to a noop. At present this
includes support for sparse matrix data, but the implementation has
not been optimized for sparse data at this time and so should be used
with caution.
Instructions for coercing common image formats into some
AbstractVector{<:Image}
are
here.
Warm restart
MLJ machines cache state enabling the "warm restart" of model
training, as demonstrated in the example above. In the case of MLJFlux
models, fit!(mach)
will use a warm restart if:

only
model.epochs
has changed since the last call; or 
only
model.epochs
ormodel.optimiser
have changed since the last call andmodel.optimiser_changes_trigger_retraining == false
(the default) (the "state" part of the optimiser is ignored in this comparison). This allows one to dynamically modify learning rates, for example.
Here model=mach.model
is the associated MLJ model.
The warm restart feature makes it possible to apply early stopping criteria, as defined in EarlyStopping.jl. For an example, see /examples/mnist/. (Eventually, this will be handled by an MLJ model wrapper for controlling arbitrary iterative models.)
Training on a GPU
When instantiating a model for training on a GPU, specify
acceleration=CUDALibs()
, as in
using MLJ
ImageClassifier = @load ImageClassifier
model = ImageClassifier(epochs=10, acceleration=CUDALibs())
mach = machine(model, X, y) > fit!
In this example, the data X, y
is copied onto the GPU under the hood
on the call to fit!
and cached for use in any warm restart (see
above). The Flux chain used in training is always copied back to the
CPU at then conclusion of fit!
, and made available as
fitted_params(mach)
.
Builtin builders
MLJ provides two simple builders out of the box:

MLJFlux.Linear(σ=...)
builds a fully connected two layer network withn_in
inputs andn_out
outputs, with activation functionσ
, defaulting to aMLJFlux.relu
. 
MLJFlux.Short(n_hidden=..., dropout=..., σ=...)
builds a fullconnected threelayer network withn_in
inputs andn_out
outputs usingn_hidden
nodes in the hidden layer and the specifieddropout
(defaulting to 0.5). An activation functionσ
is applied between the hidden and final layers. Ifn_hidden=0
(the default) thenn_hidden
is the geometric mean of the number of input and output nodes.
See Table 1 above to see how n_in
and n_out
relate to the data.
Model hyperparameters.
All models share the following hyperparameters:

builder
: Default =MLJFlux.Linear(σ=Flux.relu)
(regressors) orMLJFlux.Short(n_hidden=0, dropout=0.5, σ=Flux.σ)
(classifiers) 
optimiser
: The optimiser to use for training. Default =Flux.ADAM()

loss
: The loss function used for training. Default =Flux.mse
(regressors) andFlux.crossentropy
(classifiers) 
n_epochs
: Number of epochs to train for. Default =10

batch_size
: The batch_size for the data. Default = 1 
lambda
: The regularization strength. Default = 0. Range = [0, ∞) 
alpha
: The L2/L1 mix of regularization. Default = 0. Range = [0, 1] 
acceleration
: UseCUDALibs()
for training on GPU; default isCPU1()
. 
optimiser_changes_trigger_retraining
: True if fitting an associated machine should trigger retraining from scratch whenever the optimiser changes. Default =false
The classifiers have an additional hyperparameter finaliser
(default
= Flux.softmax
) which is the operation applied to the unnormalized
output of the final layer to obtain probabilities (outputs summing to
one). Default = Flux.softmax
. It should return a vector of the same
length as its input.
Defining a new builder
Following is an example defining a new builder for creating a simple
fullyconnected neural network with two hidden layers, with n1
nodes
in the first hidden layer, and n2
nodes in the second, for use in
any of the first three models in Table 1. The definition includes one
mutable struct and one method:
mutable struct MyNetwork <: MLJFlux.Builder
n1 :: Int
n2 :: Int
end
function MLJFlux.build(nn::MyNetwork, n_in, n_out)
return Chain(Dense(n_in, nn.n1), Dense(nn.n1, nn.n2), Dense(nn.n2, n_out))
end
Note here that n_in
and n_out
depend on the size of the data (see
Table 1).
For a concrete image classification example, see examples/mnist.
More generally, defining a new builder means defining a new struct
subtyping MLJFlux.Builder
and defining a new MLJFlux.build
method
with one of these signatures:
MLJFlux.build(builder::MyNetwork, n_in, n_out)
MLJFlux.build(builder::MyNetwork, n_in, n_out, n_channels) # for use with `ImageClassifier`
This method must return a Flux.Chain
instance, chain
, subject to the
following conditions:

chain(x)
must make sense:
for any
x <: Vector{<:AbstractFloat}
of lengthn_in
(for use with one of the first three model types); or 
for any
x <: Array{<:Float32, 4}
of size(W, H, n_channels, batch_size)
, where(W, H) = n_in
,n_channels
is 1 or 3, andbatch_size
is any integer (for use withImageClassifier
)


The object returned by
chain(x)
must be anAbstractFloat
vector of lengthn_out
.
Loss functions
Currently, the loss function specified by loss=...
is applied
internally by Flux and needs to conform to the Flux API. You cannot,
for example, supply one of MLJ's probabilistic loss functions, such as
MLJ.cross_entropy
to one of the classifier constructors, although
you should use MLJ loss functions in MLJ metaalgorithms.
An image classification example
An expanded version of this example, with early stopping and snapshots, is available here.
We define a builder that builds a chain with six alternating convolution and maxpool layers, and a final dense layer, which we apply to the MNIST image dataset.
First we define a generic builder (working for any image size, color or gray):
using MLJ
using Flux
# helper function
function flatten(x::AbstractArray)
return reshape(x, :, size(x)[end])
end
import MLJFlux
mutable struct MyConvBuilder
filter_size::Int
channels1::Int
channels2::Int
channels3::Int
end
function MLJFlux.build(b::MyConvBuilder, n_in, n_out, n_channels)
k, c1, c2, c3 = b.filter_size, b.channels1, b.channels2, b.channels3
mod(k, 2) == 1  error("`filter_size` must be odd. ")
# padding to preserve image size on convolution:
p = div(k  1, 2)
# compute size, in first two dims, of output of final maxpool layer:
half(x) = div(x, 2)
h = n_in[1] > half > half > half
w = n_in[2] > half > half > half
return Chain(
Conv((k, k), n_channels => c1, pad=(p, p), relu),
MaxPool((2, 2)),
Conv((k, k), c1 => c2, pad=(p, p), relu),
MaxPool((2, 2)),
Conv((k, k), c2 => c3, pad=(p, p), relu),
MaxPool((2 ,2)),
flatten,
Dense(h*w*c3, n_out))
end
Next, we load some of the MNIST data and check scientific types conform to those is the table above:
N = 1000
X, y = Flux.Data.MNIST.images()[1:N], Flux.Data.MNIST.labels()[1:N];
julia> scitype(X)
AbstractArray{GrayImage{28,28},1}
julia> scitype(y)
AbstractArray{Count,1}
For classifiers, target must have element scitype <: Finite
, so we fix this:
y = coerce(y, Multiclass);
Instantiating an image classifier model:
ImageClassifier = @load ImageClassifier
clf = ImageClassifier(builder=MyConvBuilder(3, 16, 32, 32),
epochs=10,
loss=Flux.crossentropy)
And evaluating the accuracy of the model on a 30% holdout set:
mach = machine(clf, X, y)
julia> evaluate!(mach,
resampling=Holdout(rng=123, fraction_train=0.7),
operation=predict_mode,
measure=misclassification_rate)
┌────────────────────────┬───────────────┬────────────┐
│ _.measure │ _.measurement │ _.per_fold │
├────────────────────────┼───────────────┼────────────┤
│ misclassification_rate │ 0.0467 │ [0.0467] │
└────────────────────────┴───────────────┴────────────┘