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NeuralNetworkClassifier

NeuralNetworkClassifier

A model type for constructing a neural network classifier, based on MLJFlux.jl, and implementing the MLJ model interface.

From MLJ, the type can be imported using

NeuralNetworkClassifier = @load NeuralNetworkClassifier pkg=MLJFlux

Do model = NeuralNetworkClassifier() to construct an instance with default hyper-parameters. Provide keyword arguments to override hyper-parameter defaults, as in NeuralNetworkClassifier(builder=...).

NeuralNetworkClassifier is for training a data-dependent Flux.jl neural network for making probabilistic predictions of a Multiclass or OrderedFactor target, given a table of Continuous features. Users provide a recipe for constructing the network, based on properties of the data that is encountered, by specifying an appropriate builder. See MLJFlux documentation for more on builders.

In addition to features with Continuous scientific element type, this model supports categorical features in the input table. If present, such features are embedded into dense vectors by the use of an additional EntityEmbedderLayer layer after the input, as described in Entity Embeddings of Categorical Variables by Cheng Guo, Felix Berkhahn arXiv, 2016.

Training data

In MLJ or MLJBase, bind an instance model to data with

mach = machine(model, X, y)

Here:

  • X provides input features and is either: (i) a Matrix with Continuous element scitype (typically Float32); or (ii) a table of input features (eg, a DataFrame) whose columns have Continuous, Multiclass or OrderedFactor element scitype; check column scitypes with schema(X). If any Multiclass or OrderedFactor features appear, the constructed network will use an EntityEmbedderLayer layer to transform them into dense vectors. If X is a Matrix, it is assumed that columns correspond to features and rows corresponding to observations.
  • y is the target, which can be any AbstractVector whose element scitype is Multiclass or OrderedFactor; check the scitype with scitype(y)

Train the machine with fit!(mach, rows=...).

Hyper-parameters

  • builder=MLJFlux.Short(): An MLJFlux builder that constructs a neural network. Possible builders include: MLJFlux.Linear, MLJFlux.Short, and MLJFlux.MLP. See MLJFlux.jl documentation for examples of user-defined builders. See also finaliser below.

  • optimiser::Optimisers.Adam(): An Optimisers.jl optimiser. The optimiser performs the updating of the weights of the network. To choose a learning rate (the update rate of the optimizer), a good rule of thumb is to start out at 10e-3, and tune using powers of 10 between 1 and 1e-7.

  • loss=Flux.crossentropy: The loss function which the network will optimize. Should be a function which can be called in the form loss(yhat, y). Possible loss functions are listed in the Flux loss function documentation. For a classification task, the most natural loss functions are:

    • Flux.crossentropy: Standard multiclass classification loss, also known as the log loss.
    • Flux.logitcrossentopy: Mathematically equal to crossentropy, but numerically more stable than finalising the outputs with softmax and then calculating crossentropy. You will need to specify finaliser=identity to remove MLJFlux's default softmax finaliser, and understand that the output of predict is then unnormalized (no longer probabilistic).
    • Flux.tversky_loss: Used with imbalanced data to give more weight to false negatives.
    • Flux.focal_loss: Used with highly imbalanced data. Weights harder examples more than easier examples.

    Currently MLJ measures are not supported values of loss.

  • epochs::Int=10: The duration of training, in epochs. Typically, one epoch represents one pass through the complete the training dataset.

  • batch_size::int=1: the batch size to be used for training, representing the number of samples per update of the network weights.] Typically, batch size is between 8 and 512. Increassing batch size may accelerate training if acceleration=CUDALibs() and a GPU is available.

  • lambda::Float64=0: The strength of the weight regularization penalty. Can be any value in the range [0, ∞). Note the history reports unpenalized losses.

  • alpha::Float64=0: The L2/L1 mix of regularization, in the range [0, 1]. A value of 0 represents L2 regularization, and a value of 1 represents L1 regularization.

  • rng::Union{AbstractRNG, Int64}: The random number generator or seed used during training. The default is Random.default_rng().

  • optimizer_changes_trigger_retraining::Bool=false: Defines what happens when re-fitting a machine if the associated optimiser has changed. If true, the associated machine will retrain from scratch on fit! call, otherwise it will not.

  • acceleration::AbstractResource=CPU1(): Defines on what hardware training is done. For Training on GPU, use CUDALibs().

  • finaliser=Flux.softmax: The final activation function of the neural network (applied after the network defined by builder). Defaults to Flux.softmax.

  • embedding_dims: a Dict whose keys are names of categorical features, given as symbols, and whose values are numbers representing the desired dimensionality of the entity embeddings of such features: an integer value of 7, say, sets the embedding dimensionality to 7; a float value of 0.5, say, sets the embedding dimensionality to ceil(0.5 * c), where c is the number of feature levels. Unspecified feature dimensionality defaults to min(c - 1, 10).

Operations

  • predict(mach, Xnew): return predictions of the target given new features Xnew, which should have the same scitype as X above. Predictions are probabilistic but uncalibrated.
  • predict_mode(mach, Xnew): Return the modes of the probabilistic predictions returned above.
  • transform(mach, Xnew): Assuming Xnew has the same schema as X, transform the categorical features of Xnew into dense Continuous vectors using the MLJFlux.EntityEmbedderLayer layer present in the network. Does nothing in case the model was trained on an input X that lacks categorical features.

Fitted parameters

The fields of fitted_params(mach) are:

  • chain: The trained "chain" (Flux.jl model), namely the series of layers, functions, and activations which make up the neural network. This includes the final layer specified by finaliser (eg, softmax).

Report

The fields of report(mach) are:

  • training_losses: A vector of training losses (penalised if lambda != 0) in historical order, of length epochs + 1. The first element is the pre-training loss.

Examples

In this example we build a classification model using the Iris dataset. This is a very basic example, using a default builder and no standardization. For a more advanced illustration, see NeuralNetworkRegressor or ImageClassifier, and examples in the MLJFlux.jl documentation.

using MLJ
using Flux
import RDatasets
import Optimisers

First, we can load the data:

iris = RDatasets.dataset("datasets", "iris");
y, X = unpack(iris, ==(:Species), rng=123); ## a vector and a table
NeuralNetworkClassifier = @load NeuralNetworkClassifier pkg=MLJFlux
clf = NeuralNetworkClassifier()

Next, we can train the model:

mach = machine(clf, X, y)
fit!(mach)

We can train the model in an incremental fashion, altering the learning rate as we go, provided optimizer_changes_trigger_retraining is false (the default). Here, we also change the number of (total) iterations:

clf.optimiser = Optimisers.Adam(clf.optimiser.eta * 2)
clf.epochs = clf.epochs + 5

fit!(mach, verbosity=2) ## trains 5 more epochs

We can inspect the mean training loss using the cross_entropy function:

training_loss = cross_entropy(predict(mach, X), y)

And we can access the Flux chain (model) using fitted_params:

chain = fitted_params(mach).chain

Finally, we can see how the out-of-sample performance changes over time, using MLJ's learning_curve function:

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")

See also ImageClassifier, NeuralNetworkBinaryClassifier.