model_utils.py 4.33 KB
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# @author lucasmiranda42

from keras import backend as K
from tensorflow.keras.constraints import Constraint
from tensorflow.keras.layers import Layer
import tensorflow as tf


# Helper functions
def sampling(args, epsilon_std=1.0):
    z_mean, z_log_sigma = args
    epsilon = K.random_normal(shape=K.shape(z_mean), mean=0.0, stddev=epsilon_std)
    return z_mean + K.exp(z_log_sigma) * epsilon


def compute_kernel(x, y):
    x_size = K.shape(x)[0]
    y_size = K.shape(y)[0]
    dim = K.shape(x)[1]
    tiled_x = K.tile(K.reshape(x, K.stack([x_size, 1, dim])), K.stack([1, y_size, 1]))
    tiled_y = K.tile(K.reshape(y, K.stack([1, y_size, dim])), K.stack([x_size, 1, 1]))
    return K.exp(
        -tf.reduce_mean(K.square(tiled_x - tiled_y), axis=2) / K.cast(dim, tf.float32)
    )


def compute_mmd(x, y):
    x_kernel = compute_kernel(x, x)
    y_kernel = compute_kernel(y, y)
    xy_kernel = compute_kernel(x, y)
    return (
        tf.reduce_mean(x_kernel)
        + tf.reduce_mean(y_kernel)
        - 2 * tf.reduce_mean(xy_kernel)
    )


# Custom layers for efficiency/losses
class DenseTranspose(Layer):
    def __init__(self, dense, output_dim, activation=None, **kwargs):
        self.dense = dense
        self.output_dim = output_dim
        self.activation = tf.keras.activations.get(activation)
        super().__init__(**kwargs)

    def get_config(self):
        config = super().get_config().copy()
        config.update(
            {
                "dense": self.dense,
                "output_dim": self.output_dim,
                "activation": self.activation,
            }
        )
        return config

    def build(self, batch_input_shape):
        self.biases = self.add_weight(
            name="bias", shape=[self.dense.input_shape[-1]], initializer="zeros"
        )
        super().build(batch_input_shape)

    def call(self, inputs, **kwargs):
        z = tf.matmul(inputs, self.dense.weights[0], transpose_b=True)
        return self.activation(z + self.biases)

    def compute_output_shape(self, input_shape):
        return input_shape[0], self.output_dim


class UncorrelatedFeaturesConstraint(Constraint):
    def __init__(self, encoding_dim, weightage=1.0):
        self.encoding_dim = encoding_dim
        self.weightage = weightage

    def get_config(self):

        config = super().get_config().copy()
        config.update(
            {"encoding_dim": self.encoding_dim, "weightage": self.weightage,}
        )
        return config

    def get_covariance(self, x):
        x_centered_list = []

        for i in range(self.encoding_dim):
            x_centered_list.append(x[:, i] - K.mean(x[:, i]))

        x_centered = tf.stack(x_centered_list)
        covariance = K.dot(x_centered, K.transpose(x_centered)) / tf.cast(
            x_centered.get_shape()[0], tf.float32
        )

        return covariance

    # Constraint penalty
    def uncorrelated_feature(self, x):
        if self.encoding_dim <= 1:
            return 0.0
        else:
            output = K.sum(
                K.square(
                    self.covariance
                    - tf.math.multiply(self.covariance, K.eye(self.encoding_dim))
                )
            )
            return output

    def __call__(self, x):
        self.covariance = self.get_covariance(x)
        return self.weightage * self.uncorrelated_feature(x)


class KLDivergenceLayer(Layer):

    """ Identity transform layer that adds KL divergence
    to the final model loss.
    """

    def __init__(self, *args, **kwargs):
        self.is_placeholder = True
        super(KLDivergenceLayer, self).__init__(*args, **kwargs)

    def call(self, inputs, **kwargs):

        mu, log_var = inputs

        kl_batch = -0.5 * K.sum(1 + log_var - K.square(mu) - K.exp(log_var), axis=-1)

        self.add_loss(K.mean(kl_batch), inputs=inputs)

        return inputs


class MMDiscrepancyLayer(Layer):
    """ Identity transform layer that adds MM discrepancy
    to the final model loss.
    """

    def __init__(self, *args, **kwargs):
        self.is_placeholder = True
        super(MMDiscrepancyLayer, self).__init__(*args, **kwargs)

    def call(self, z, **kwargs):
        true_samples = K.random_normal(
            K.shape(z), mean=0.0, stddev=2.0 / K.cast_to_floatx(K.shape(z)[1])
        )
        mmd_batch = compute_mmd(z, true_samples)

        self.add_loss(K.mean(mmd_batch), inputs=z)

        return z