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Neural Networks

Concrete ML provides simple built-in neural networks models with a scikit-learn interface through the NeuralNetClassifier and NeuralNetRegressor classes.

Concrete ML scikit-learn
NeuralNetClassifier MLPClassifier
NeuralNetRegressor MLPRegressor

The neural network models are implemented with skorch, which provides a scikit-learn-like interface to Torch models (more here).

Concrete ML models are multi-layers, fully-connected networks with customizable activation functions and a number of neurons in each layer. This approach is similar to what is available in scikit-learn using the MLPClassifier/MLPRegressor classes. The built-in models train easily with a single call to .fit(), which will automatically quantize weights and activations. These models use Quantization Aware Training, allowing good performance for low precision (down to 2-3 bits) weights and activations.

While NeuralNetClassifier and NeuralNetClassifier provide scikit-learn-like models, their architecture is somewhat restricted to make training easy and robust. If you need more advanced models, you can convert custom neural networks as described in the FHE-friendly models documentation.

{% hint style="warning" %} Good quantization parameter values are critical to make models respect FHE constraints. Weights and activations should be quantized to low precision (e.g., 2-4 bits). The sparsity of the network can be tuned as described below to avoid accumulator overflow. {% endhint %}

Example usage

To create an instance of a Fully Connected Neural Network (FCNN), you need to instantiate one of the NeuralNetClassifier and NeuralNetRegressor classes and configure a number of parameters that are passed to their constructor. Note that some parameters need to be prefixed by module__, while others don't. The parameters related to the model (i.e., the underlying nn.Module), must have the prefix. The parameters related to training options do not require the prefix.

from concrete.ml.sklearn import NeuralNetClassifier
import torch.nn as nn

n_inputs = 10
n_outputs = 2
params = {
    "module__n_layers": 2,
    "module__n_w_bits": 2,
    "module__n_a_bits": 2,
    "module__n_accum_bits": 8,
    "module__n_hidden_neurons_multiplier": 1,
    "module__activation_function": nn.ReLU,
    "max_epochs": 10,
}

concrete_classifier = NeuralNetClassifier(**params)

The Classifier Comparison notebook shows the behavior of built-in neural networks on several synthetic data-sets.

Comparison neural networks

The figure above right shows the Concrete ML neural network, trained with Quantization Aware Training in a FHE-compatible configuration. The figure compares this network to the floating-point equivalent, trained with scikit-learn.

Architecture parameters

  • module__n_layers: number of layers in the FCNN, must be at least 1. Note that this is the total number of layers. For a single, hidden layer NN model, set module__n_layers=2
  • module__activation_function: can be one of the Torch activations (e.g., nn.ReLU, see the full list here)

Quantization parameters

  • n_w_bits (default 3): number of bits for weights
  • n_a_bits (default 3): number of bits for activations and inputs
  • n_accum_bits (default 8): maximum accumulator bit-width that is desired. The implementation will attempt to keep accumulators under this bit-width through pruning (i.e., setting some weights to zero)

Training parameters (from skorch)

  • max_epochs: The number of epochs to train the network (default 10)
  • verbose: Whether to log loss/metrics during training (default: False)
  • lr: Learning rate (default 0.001)

Other parameters from skorch can be found in the skorch documentation.

Advanced parameters

  • module__n_hidden_neurons_multiplier: The number of hidden neurons will be automatically set proportional to the dimensionality of the input. This parameter controls the proportionality factor and is set to 4 by default. This value gives good accuracy while avoiding accumulator overflow. See the pruning and quantization sections for more info.

Class weights

You can give weights to each class to use in training. Note that this must be supported by the underlying PyTorch loss function.

    from sklearn.utils.class_weight import compute_class_weight
    params["criterion__weight"] = compute_class_weight("balanced", classes=classes, y=y_train)

Overflow errors

The n_hidden_neurons_multiplier parameter influences training accuracy as it controls the number of non-zero neurons that are allowed in each layer. Increasing n_hidden_neurons_multiplier improves accuracy, but should take into account precision limitations to avoid overflow in the accumulator. The default value is a good compromise that avoids overflow in most cases, but you may want to change the value of this parameter to reduce the breadth of the network if you have overflow errors. A value of 1 should be completely safe with respect to overflow.