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Using ONNX

In addition to Concrete ML models and custom models in torch, it is also possible to directly compile ONNX models. This can be particularly appealing, notably to import models trained with Keras.

ONNX models can be compiled by directly importing models that are already quantized with Quantization Aware Training (QAT) or by performing Post-Training Quantization (PTQ) with Concrete ML.

Simple example

The following example shows how to compile an ONNX model using PTQ. The model was initially trained using Keras before being exported to ONNX. The training code is not shown here.

This example uses Post-Training Quantization, i.e., the quantization is not performed during training. This model would not have good performance in FHE. Quantization Aware Training should be added by the model developer. Additionally, importing QAT ONNX models can be done as shown below.

import numpy
import onnx
import tensorflow
import tf2onnx

from concrete.ml.torch.compile import compile_onnx_model
from concrete.fhe.compilation import Configuration


class FC(tensorflow.keras.Model):
    """A fully-connected model."""

    def __init__(self):
        super().__init__()
        hidden_layer_size = 10
        output_size = 5

        self.dense1 = tensorflow.keras.layers.Dense(
            hidden_layer_size,
            activation=tensorflow.nn.relu,
        )
        self.dense2 = tensorflow.keras.layers.Dense(output_size, activation=tensorflow.nn.relu6)
        self.flatten = tensorflow.keras.layers.Flatten()

    def call(self, inputs):
        """Forward function."""
        x = self.flatten(inputs)
        x = self.dense1(x)
        x = self.dense2(x)
        return self.flatten(x)


n_bits = 6
input_output_feature = 2
input_shape = (input_output_feature,)
num_inputs = 1
n_examples = 5000

# Define the Keras model
keras_model = FC()
keras_model.build((None,) + input_shape)
keras_model.compute_output_shape(input_shape=(None, input_output_feature))

# Create random input
input_set = numpy.random.uniform(-100, 100, size=(n_examples, *input_shape))

# Convert to ONNX
tf2onnx.convert.from_keras(keras_model, opset=14, output_path="tmp.model.onnx")

onnx_model = onnx.load("tmp.model.onnx")
onnx.checker.check_model(onnx_model)

# Compile
quantized_module = compile_onnx_model(
    onnx_model, input_set, n_bits=2
)

# Create test data from the same distribution and quantize using
# learned quantization parameters during compilation
x_test = tuple(numpy.random.uniform(-100, 100, size=(1, *input_shape)) for _ in range(num_inputs))

y_clear = quantized_module.forward(*x_test, fhe="disable")
y_fhe = quantized_module.forward(*x_test, fhe="execute")

print("Execution in clear: ", y_clear)
print("Execution in FHE:   ", y_fhe)
print("Equality:           ", numpy.sum(y_clear == y_fhe), "over", numpy.size(y_fhe), "values")

While Keras was used in this example, it is not officially supported. Additional work is needed to test all of Keras's types of layers and models.

Quantization Aware Training

Models trained using Quantization Aware Training contain quantizers in the ONNX graph. These quantizers ensure that the inputs to the Linear/Dense and Conv layers are quantized. Since these QAT models have quantizers that are configured during training to a specific number of bits, the ONNX graph will need to be imported using the same settings:

# Define the number of bits to use for quantizing weights and activations during training
n_bits_qat = 3  

quantized_numpy_module = compile_onnx_model(
    onnx_model,
    input_set,
    import_qat=True,
    n_bits=n_bits_qat,
)

Supported operators

The following operators are supported for evaluation and conversion to an equivalent FHE circuit. Other operators were not implemented, either due to FHE constraints or because they are rarely used in PyTorch activations or scikit-learn models.

Abs
Acos
Acosh
Add
Asin
Asinh
Atan
Atanh
AveragePool
BatchNormalization
Cast
Celu
Clip
Concat
Constant
ConstantOfShape
Conv
Cos
Cosh
Div
Elu
Equal
Erf
Exp
Flatten
Floor
Gather
Gemm
Greater
GreaterOrEqual
HardSigmoid
HardSwish
Identity
LeakyRelu
Less
LessOrEqual
Log
MatMul
Max
MaxPool
Min
Mul
Neg
Not
Or
PRelu
Pad
Pow
ReduceSum
Relu
Reshape
Round
Selu
Shape
Sigmoid
Sign
Sin
Sinh
Slice
Softplus
Squeeze
Sub
Tan
Tanh
ThresholdedRelu
Transpose
Unsqueeze
Where
onnx.brevitas.Quant