init.py

import math
import numpy as np

import paddle


def _calculate_fan_in_and_fan_out(tensor):
    dimensions = len(tensor.shape)
    if dimensions < 2:
        raise ValueError(
            "Fan in and fan out can not be computed for tensor with fewer than 2 dimensions"
        )

    num_input_fmaps = tensor.shape[1]
    num_output_fmaps = tensor.shape[0]
    receptive_field_size = 1
    if len(tensor.shape) > 2:
        receptive_field_size = paddle.numel(tensor[0][0])
    fan_in = num_input_fmaps * receptive_field_size
    fan_out = num_output_fmaps * receptive_field_size

    return fan_in, fan_out


def _calculate_correct_fan(tensor, mode):
    mode = mode.lower()
    valid_modes = ['fan_in', 'fan_out']
    if mode not in valid_modes:
        raise ValueError("Mode {} not supported, please use one of {}".format(
            mode, valid_modes))

    fan_in, fan_out = _calculate_fan_in_and_fan_out(tensor)
    return fan_in if mode == 'fan_in' else fan_out


def calculate_gain(nonlinearity, param=None):
    """Return the recommended gain value for the given nonlinearity function.
    The values are as follows:

    ================= ====================================================
    nonlinearity      gain
    ================= ====================================================
    Linear / Identity :math:`1`
    Conv{1,2,3}D      :math:`1`
    Sigmoid           :math:`1`
    Tanh              :math:`\frac{5}{3}`
    ReLU              :math:`\sqrt{2}`
    Leaky Relu        :math:`\sqrt{\frac{2}{1 + \text{negative\_slope}^2}}`
    ================= ====================================================

    Args:
        nonlinearity: the non-linear function (`nn.functional` name)
        param: optional parameter for the non-linear function

    """
    linear_fns = [
        'linear', 'conv1d', 'conv2d', 'conv3d', 'conv_transpose1d',
        'conv_transpose2d', 'conv_transpose3d'
    ]
    if nonlinearity in linear_fns or nonlinearity == 'sigmoid':
        return 1
    elif nonlinearity == 'tanh':
        return 5.0 / 3
    elif nonlinearity == 'relu':
        return math.sqrt(2.0)
    elif nonlinearity == 'leaky_relu':
        if param is None:
            negative_slope = 0.01
        elif not isinstance(param, bool) and isinstance(
                param, int) or isinstance(param, float):
            # True/False are instances of int, hence check above
            negative_slope = param
        else:
            raise ValueError(
                "negative_slope {} not a valid number".format(param))
        return math.sqrt(2.0 / (1 + negative_slope**2))
    else:
        raise ValueError("Unsupported nonlinearity {}".format(nonlinearity))


@paddle.no_grad()
def constant_(x, value):
    temp_value = value * paddle.ones(x.shape, x.dtype)
    x.set_value(temp_value)
    return x


@paddle.no_grad()
def normal_(x, mean=0., std=1.):
    temp_value = paddle.normal(mean, std, shape=x.shape)
    x.set_value(temp_value)
    return x


@paddle.no_grad()
def uniform_(x, a=-1., b=1.):
    temp_value = paddle.uniform(min=a, max=b, shape=x.shape)
    x.set_value(temp_value)
    return x


@paddle.no_grad()
def xavier_uniform_(x, gain=1.):
    """Fills the input `Tensor` with values according to the method
    described in `Understanding the difficulty of training deep feedforward
    neural networks` - Glorot, X. & Bengio, Y. (2010), using a uniform
    distribution. The resulting tensor will have values sampled from
    :math:`\mathcal{U}(-a, a)` where

    .. math::
        a = \text{gain} \times \sqrt{\frac{6}{\text{fan\_in} + \text{fan\_out}}}

    Also known as Glorot initialization.

    Args:
        x: an n-dimensional `paddle.Tensor`
        gain: an optional scaling factor

    """
    fan_in, fan_out = _calculate_fan_in_and_fan_out(x)
    std = gain * math.sqrt(2.0 / float(fan_in + fan_out))
    a = math.sqrt(3.0) * std  # Calculate uniform bounds from standard deviation

    return uniform_(x, -a, a)


@paddle.no_grad()
def xavier_normal_(x, gain=1.):
    """Fills the input `Tensor` with values according to the method
    described in `Understanding the difficulty of training deep feedforward
    neural networks` - Glorot, X. & Bengio, Y. (2010), using a normal
    distribution. The resulting tensor will have values sampled from
    :math:`\mathcal{N}(0, \text{std}^2)` where

    .. math::
        \text{std} = \text{gain} \times \sqrt{\frac{2}{\text{fan\_in} + \text{fan\_out}}}

    Also known as Glorot initialization.

    Args:
        tensor: an n-dimensional `paddle.Tensor`
        gain: an optional scaling factor

    """
    fan_in, fan_out = _calculate_fan_in_and_fan_out(x)
    std = gain * math.sqrt(2.0 / float(fan_in + fan_out))

    return normal_(x, 0., std)


@paddle.no_grad()
def kaiming_uniform_(x, a=0, mode='fan_in', nonlinearity='leaky_relu'):
    """Fills the input `Tensor` with values according to the method
    described in `Delving deep into rectifiers: Surpassing human-level
    performance on ImageNet classification` - He, K. et al. (2015), using a
    uniform distribution. The resulting tensor will have values sampled from
    :math:`\mathcal{U}(-\text{bound}, \text{bound})` where

    .. math::
        \text{bound} = \text{gain} \times \sqrt{\frac{3}{\text{fan\_mode}}}

    Also known as He initialization.

    Args:
        x: an n-dimensional `paddle.Tensor`
        a: the negative slope of the rectifier used after this layer (only
            used with ``'leaky_relu'``)
        mode: either ``'fan_in'`` (default) or ``'fan_out'``. Choosing ``'fan_in'``
            preserves the magnitude of the variance of the weights in the
            forward pass. Choosing ``'fan_out'`` preserves the magnitudes in the
            backwards pass.
        nonlinearity: the non-linear function (`nn.functional` name),
            recommended to use only with ``'relu'`` or ``'leaky_relu'`` (default).

    """
    fan = _calculate_correct_fan(x, mode)
    gain = calculate_gain(nonlinearity, a)
    std = gain / math.sqrt(fan)
    bound = math.sqrt(
        3.0) * std  # Calculate uniform bounds from standard deviation

    temp_value = paddle.uniform(x.shape, min=-bound, max=bound)
    x.set_value(temp_value)

    return x


@paddle.no_grad()
def kaiming_normal_(x, a=0, mode='fan_in', nonlinearity='leaky_relu'):
    """Fills the input `Tensor` with values according to the method
    described in `Delving deep into rectifiers: Surpassing human-level
    performance on ImageNet classification` - He, K. et al. (2015), using a
    normal distribution. The resulting tensor will have values sampled from
    :math:`\mathcal{N}(0, \text{std}^2)` where

    .. math::
        \text{std} = \frac{\text{gain}}{\sqrt{\text{fan\_mode}}}

    Also known as He initialization.

    Args:
        x: an n-dimensional `paddle.Tensor`
        a: the negative slope of the rectifier used after this layer (only
            used with ``'leaky_relu'``)
        mode: either ``'fan_in'`` (default) or ``'fan_out'``. Choosing ``'fan_in'``
            preserves the magnitude of the variance of the weights in the
            forward pass. Choosing ``'fan_out'`` preserves the magnitudes in the
            backwards pass.
        nonlinearity: the non-linear function (`nn.functional` name),
            recommended to use only with ``'relu'`` or ``'leaky_relu'`` (default).

    """
    fan = _calculate_correct_fan(x, mode)
    gain = calculate_gain(nonlinearity, a)
    std = gain / math.sqrt(fan)

    temp_value = paddle.normal(0, std, shape=x.shape)
    x.set_value(temp_value)
    return x


def constant_init(layer, val, bias=0):
    if hasattr(layer, 'weight') and layer.weight is not None:
        constant_(layer.weight, val)
    if hasattr(layer, 'bias') and layer.bias is not None:
        constant_(layer.bias, bias)


def xavier_init(layer, gain=1, bias=0, distribution='normal'):
    assert distribution in ['uniform', 'normal']
    if distribution == 'uniform':
        xavier_uniform_(layer.weight, gain=gain)
    else:
        xavier_normal_(layer.weight, gain=gain)
    if hasattr(layer, 'bias') and layer.bias is not None:
        constant_(layer.bias, bias)


def normal_init(layer, mean=0, std=1, bias=0):
    normal_(layer.weight, mean, std)
    if hasattr(layer, 'bias') and layer.bias is not None:
        constant_(layer.bias, bias)


def uniform_init(layer, a=0, b=1, bias=0):
    uniform_(layer.weight, a, b)
    if hasattr(layer, 'bias') and layer.bias is not None:
        constant_(layer.bias, bias)


def kaiming_init(layer,
                 a=0,
                 mode='fan_out',
                 nonlinearity='relu',
                 bias=0,
                 distribution='normal'):
    assert distribution in ['uniform', 'normal']
    if distribution == 'uniform':
        kaiming_uniform_(
            layer.weight, a=a, mode=mode, nonlinearity=nonlinearity)
    else:
        kaiming_normal_(layer.weight, a=a, mode=mode, nonlinearity=nonlinearity)
    if hasattr(layer, 'bias') and layer.bias is not None:
        constant_(layer.bias, bias)


def init_weights(net, init_type='normal', init_gain=0.02):
    """Initialize network weights.
    Args:
        net (nn.Layer): network to be initialized
        init_type (str): the name of an initialization method: normal | xavier | kaiming | orthogonal
        init_gain (float): scaling factor for normal, xavier and orthogonal.
    We use 'normal' in the original pix2pix and CycleGAN paper. But xavier and kaiming might
    work better for some applications. Feel free to try yourself.
    """

    def init_func(m):  # define the initialization function
        classname = m.__class__.__name__
        if hasattr(m, 'weight') and (classname.find('Conv') != -1
                                     or classname.find('Linear') != -1):
            if init_type == 'normal':
                normal_(m.weight, 0.0, init_gain)
            elif init_type == 'xavier':
                xavier_normal_(m.weight, gain=init_gain)
            elif init_type == 'kaiming':
                kaiming_normal_(m.weight, a=0, mode='fan_in')
            else:
                raise NotImplementedError(
                    'initialization method [%s] is not implemented' % init_type)
            if hasattr(m, 'bias') and m.bias is not None:
                constant_(m.bias, 0.0)
        elif classname.find(
                'BatchNorm'
        ) != -1:  # BatchNorm Layer's weight is not a matrix; only normal distribution applies.
            normal_(m.weight, 1.0, init_gain)
            constant_(m.bias, 0.0)

    print('initialize network with %s' % init_type)
    net.apply(init_func)  # apply the initialization function <init_func>