[FIXED] Why should the training label for Generator in GAN be always True?

Issue

I am currently learning deep learning especially GAN. I found a simple code of GAN from a web site below. https://medium.com/@devnag/generative-adversarial-networks-gans-in-50-lines-of-code-pytorch-e81b79659e3f

However, in the code, I don't understand why we always need to give true label to Generator as below.

        for g_index in range(g_steps):
            # 2. Train G on D's response (but DO NOT train D on these labels)
            G.zero_grad()

            gen_input = Variable(gi_sampler(minibatch_size, g_input_size))
            g_fake_data = G(gen_input)
            dg_fake_decision = D(preprocess(g_fake_data.t()))
            g_error = criterion(dg_fake_decision, Variable(torch.ones(1)))  # we want to fool, so pretend it's all genuine

            g_error.backward()
            g_optimizer.step()  # Only optimizes G's parameters

Specifically, on this line.

            g_error = criterion(dg_fake_decision, Variable(torch.ones(1)))  # we want to fool, so pretend it's all genuine

Input data for Generator is fake data(includes noise), so if we assign True labels on those input data, I think Generator ends up creating data which is similar to fake data(doesn't look like genuine). Is my understanding wrong? Sorry for the silly question, but if you have knowledge, plz help me out. I'll put a whole code below.

    #!/usr/bin/env python

    # Generative Adversarial Networks (GAN) example in PyTorch.
    # See related blog post at https://medium.com/@devnag/generative-adversarial-networks-gans-in-50-lines-of-code-pytorch-e81b79659e3f#.sch4xgsa9
    import numpy as np
    import torch
    import torch.nn as nn
    import torch.nn.functional as F
    import torch.optim as optim
    from torch.autograd import Variable

    # Data params
    data_mean = 4
    data_stddev = 1.25

    # Model params
    g_input_size = 1     # Random noise dimension coming into generator, per output vector
    g_hidden_size = 50   # Generator complexity
    g_output_size = 1    # size of generated output vector
    d_input_size = 100   # Minibatch size - cardinality of distributions
    d_hidden_size = 50   # Discriminator complexity
    d_output_size = 1    # Single dimension for 'real' vs. 'fake'
    minibatch_size = d_input_size

    d_learning_rate = 2e-4  # 2e-4
    g_learning_rate = 2e-4
    optim_betas = (0.9, 0.999)
    num_epochs = 30000
    print_interval = 200
    d_steps = 1  # 'k' steps in the original GAN paper. Can put the discriminator on higher training freq than generator
    g_steps = 1

    # ### Uncomment only one of these
    #(name, preprocess, d_input_func) = ("Raw data", lambda data: data, lambda x: x)
    (name, preprocess, d_input_func) = ("Data and variances", lambda data: decorate_with_diffs(data, 2.0), lambda x: x * 2)

    print("Using data [%s]" % (name))

    # ##### DATA: Target data and generator input data

    def get_distribution_sampler(mu, sigma):
        return lambda n: torch.Tensor(np.random.normal(mu, sigma, (1, n)))  # Gaussian

    def get_generator_input_sampler():
        return lambda m, n: torch.rand(m, n)  # Uniform-dist data into generator, _NOT_ Gaussian

    # ##### MODELS: Generator model and discriminator model

    class Generator(nn.Module):
        def __init__(self, input_size, hidden_size, output_size):
            super(Generator, self).__init__()
            self.map1 = nn.Linear(input_size, hidden_size)
            self.map2 = nn.Linear(hidden_size, hidden_size)
            self.map3 = nn.Linear(hidden_size, output_size)

        def forward(self, x):
            x = F.elu(self.map1(x))
            x = F.sigmoid(self.map2(x))
            return self.map3(x)

    class Discriminator(nn.Module):
        def __init__(self, input_size, hidden_size, output_size):
            super(Discriminator, self).__init__()
            self.map1 = nn.Linear(input_size, hidden_size)
            self.map2 = nn.Linear(hidden_size, hidden_size)
            self.map3 = nn.Linear(hidden_size, output_size)

        def forward(self, x):
            x = F.elu(self.map1(x))
            x = F.elu(self.map2(x))
            return F.sigmoid(self.map3(x))

    def extract(v):
        return v.data.storage().tolist()

    def stats(d):
        return [np.mean(d), np.std(d)]

    def decorate_with_diffs(data, exponent):
        mean = torch.mean(data.data, 1, keepdim=True)
        mean_broadcast = torch.mul(torch.ones(data.size()), mean.tolist()[0][0])
        diffs = torch.pow(data - Variable(mean_broadcast), exponent)
        return torch.cat([data, diffs], 1)

    d_sampler = get_distribution_sampler(data_mean, data_stddev)
    gi_sampler = get_generator_input_sampler()
    G = Generator(input_size=g_input_size, hidden_size=g_hidden_size, output_size=g_output_size)
    D = Discriminator(input_size=d_input_func(d_input_size), hidden_size=d_hidden_size, output_size=d_output_size)
    criterion = nn.BCELoss()  # Binary cross entropy: http://pytorch.org/docs/nn.html#bceloss
    d_optimizer = optim.Adam(D.parameters(), lr=d_learning_rate, betas=optim_betas)
    g_optimizer = optim.Adam(G.parameters(), lr=g_learning_rate, betas=optim_betas)

    for epoch in range(num_epochs):
        for d_index in range(d_steps):
            # 1. Train D on real+fake
            D.zero_grad()

            #  1A: Train D on real
            d_real_data = Variable(d_sampler(d_input_size))
            d_real_decision = D(preprocess(d_real_data))
            d_real_error = criterion(d_real_decision, Variable(torch.ones(1)))  # ones = true
            d_real_error.backward() # compute/store gradients, but don't change params

            #  1B: Train D on fake
            d_gen_input = Variable(gi_sampler(minibatch_size, g_input_size))
            d_fake_data = G(d_gen_input).detach()  # detach to avoid training G on these labels
            d_fake_decision = D(preprocess(d_fake_data.t()))
            d_fake_error = criterion(d_fake_decision, Variable(torch.zeros(1)))  # zeros = fake
            d_fake_error.backward()
            d_optimizer.step()     # Only optimizes D's parameters; changes based on stored gradients from backward()

        for g_index in range(g_steps):
            # 2. Train G on D's response (but DO NOT train D on these labels)
            G.zero_grad()

            gen_input = Variable(gi_sampler(minibatch_size, g_input_size))
            g_fake_data = G(gen_input)
            dg_fake_decision = D(preprocess(g_fake_data.t()))
            g_error = criterion(dg_fake_decision, Variable(torch.ones(1)))  # we want to fool, so pretend it's all genuine

            g_error.backward()
            g_optimizer.step()  # Only optimizes G's parameters

        if epoch % print_interval == 0:
            print("%s: D: %s/%s G: %s (Real: %s, Fake: %s) " % (epoch,
                                                                extract(d_real_error)[0],
                                                                extract(d_fake_error)[0],
                                                                extract(g_error)[0],
                                                                stats(extract(d_real_data)),
                                                                stats(extract(d_fake_data))))

Solution

In this part of the code you are training G to fool D, so G generates fake data and asks D whether it thinks it's real (true labels), D's gradients are then propogated all the way to G (this is possible as D's input was G's output) so that it will learn to better fool D in the next iteration.

The inputs of G are not trainable and G only tries to transform them into real data (data similar to what d_sampler generates)

Answered By - Eyal Shulman