CS231N Assignment3 入门笔记（Q4 GANs）

斯坦福2023年春季CS231N课程第三次作业（最后一次）解析、笔记与代码，作为初学者入门学习。

在这项作业中，将实现语言网络，并将其应用于 COCO 数据集上的图像标题。然后将训练生成对抗网络，生成与训练数据集相似的图像。最后，将学习自我监督学习，自动学习无标签数据集的视觉表示。

本作业的目标如下：
1.理解并实现 RNN 和 Transformer 网络。将它们与 CNN 网络相结合，为图像添加标题。
2.了解如何训练和实现生成对抗网络（GAN），以生成与数据集中的样本相似的图像。
3.了解如何利用自我监督学习技术来帮助完成图像分类任务。

Q4: Generative Adversarial Networks (GANs)

In the notebook Generative_Adversarial_Networks.ipynb you will learn how to generate images that match a training dataset and use these models to improve classifier performance when training on a large amount of unlabeled data and a small amount of labeled data. When first opening the notebook, go to Runtime > Change runtime type and set Hardware accelerator to GPU.

到目前为止，在CS231N中，我们所探索的所有神经网络的应用都是**discriminative models**，它们接受输入并被训练产生标记的输出。这包括从图像类别的直接分类到句子生成（这仍然是一个分类问题，我们在词汇空间中重复捕获多个单词标签）。在这个notebook中，我们将进行扩展，使用神经网络建立**生成性模型**。具体地说，我们将学习如何构建模型，以生成类似于一组训练图像的新图像。

### What is a GAN?

为了优化这个Minimax博弈，我们将交替在G的目标上采用梯度*下降*和在D的目标上采用梯度*上升*：

1. 更新**生成器**(G)以最小化鉴别器做出正确选择的概率。

2. 更新**鉴别器**(D)，以最大化鉴别器做出正确选择的概率。

虽然这些更新对分析很有用，但在实践中表现不佳。相反，当我们更新生成器时，我们将使用一个不同的目标：最大化**鉴别器做出不正确选择**的概率。这一微小的变化有助于消除当鉴别器有信心时生成器梯度消失的问题。这是大多数GaN论文中使用的标准更新，在此任务中，我们将交替进行以下更新：

In this assignment, we will alternate the following updates:

1. Update the generator (G) to maximize the probability of the discriminator making the incorrect choice on generated data:

更新生成器(G)以最大化鉴别器对生成数据做出错误选择的概率

2. Update the discriminator (D), to maximize the probability of the discriminator making the correct choice on real and generated data:

更新鉴别器(D)，以最大化鉴别器对真实数据和生成数据做出正确选择的概率

sample noise函数

def sample_noise(batch_size, dim, seed=None):
    """
    Generate a PyTorch Tensor of uniform random noise.

    Input:
    - batch_size: Integer giving the batch size of noise to generate.
    - dim: Integer giving the dimension of noise to generate.

    Output:
    - A PyTorch Tensor of shape (batch_size, dim) containing uniform
      random noise in the range (-1, 1).
    """
    if seed is not None:
        torch.manual_seed(seed)

    # *****START OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****

    noise = 2 * torch.rand(batch_size, dim) - 1
    return noise

    # *****END OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****

# Discriminator

def discriminator(seed=None):
    """
    Build and return a PyTorch model implementing the architecture above.
    """

    if seed is not None:
        torch.manual_seed(seed)
    model = None

    # TODO: Implement architecture                                               #
    # HINT: nn.Sequential might be helpful. You'll start by calling Flatten().   #

    model = nn.Sequential(
      nn.Flatten(),
      nn.Linear(784, 256, bias=True),
      nn.LeakyReLU(negative_slope=0.01),
      nn.Linear(256, 256, bias=True),
      nn.LeakyReLU(negative_slope=0.01),
      nn.Linear(256, 1, bias=True),
    )

    return model

# Generator

def generator(noise_dim=NOISE_DIM, seed=None):
    """
    Build and return a PyTorch model implementing the architecture above.
    """

    if seed is not None:
        torch.manual_seed(seed)

    model = None

    # TODO: Implement architecture                                               #
    # HINT: nn.Sequential might be helpful.                                      #

    model = nn.Sequential(

      nn.Linear(noise_dim, 1024, bias=True),
      nn.ReLU(),
      nn.Linear(1024, 1024, bias=True),
      nn.ReLU(),
      nn.Linear(1024, 784, bias=True),
      nn.Tanh()
    )

    return model

# GAN Loss

def bce_loss(input, target):
    """
    Numerically stable version of the binary cross-entropy loss function in PyTorch.

    Inputs:
    - input: PyTorch Tensor of shape (N, ) giving scores.
    - target: PyTorch Tensor of shape (N,) containing 0 and 1 giving targets.

    Returns:
    - A PyTorch Tensor containing the mean BCE loss over the minibatch of input data.
    """
    bce = nn.BCEWithLogitsLoss()
    return bce(input.squeeze(), target)

def discriminator_loss(logits_real, logits_fake):
    """
    Computes the discriminator loss described above.

    Inputs:
    - logits_real: PyTorch Tensor of shape (N,) giving scores for the real data.
    - logits_fake: PyTorch Tensor of shape (N,) giving scores for the fake data.

    Returns:
    - loss: PyTorch Tensor containing (scalar) the loss for the discriminator.
    """
    loss = None
    # *****START OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****

    # true张量全0,false张量全1
    true_labels = torch.ones(logits_real.shape[0]).type(dtype)
    false_labels = torch.zeros(logits_fake.shape[0]).type(dtype)
    # 将真实数据和生成数据的分数张量连接在一起，得到形状为(2N,)的张量
    targets = torch.cat([true_labels, false_labels], dim=0)
    logits = torch.cat([logits_real, logits_fake], dim=0)

    loss = bce_loss(logits, targets) * 2

    # *****END OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****
    return loss

def generator_loss(logits_fake):
    """
    Computes the generator loss described above.

    Inputs:
    - logits_fake: PyTorch Tensor of shape (N,) giving scores for the fake data.

    Returns:
    - loss: PyTorch Tensor containing the (scalar) loss for the generator.
    """
    loss = None
    # *****START OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****

    targets = torch.ones(logits_fake.shape[0]).type(dtype)

    loss = bce_loss(logits_fake, targets)

    # *****END OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****
    return loss

# Optimizing our Loss

optimizer = optim.Adam(params=model.parameters(), lr=1e-3, betas=(0.5, 0.99))

建议看一看run_a_gan函数，解析如下：

def run_a_gan(D, G, D_solver, G_solver, discriminator_loss, generator_loss, loader_train, show_every=250,
              batch_size=128, noise_size=96, num_epochs=10):
    """
    Train a GAN!

    Inputs:
    - D, G: PyTorch models for the discriminator and generator
    - D_solver, G_solver: torch.optim Optimizers to use for training the
      discriminator and generator.
    - discriminator_loss, generator_loss: Functions to use for computing the generator and
      discriminator loss, respectively.
    - show_every: Show samples after every show_every iterations.
    - batch_size: Batch size to use for training.
    - noise_size: Dimension of the noise to use as input to the generator.
    - num_epochs: Number of epochs over the training dataset to use for training.
    """
    images = []
    iter_count = 0
    # 外层循环控制迭代次数
    # 内层循环遍历训练数据加载器loader_train中的批量数据
    for epoch in range(num_epochs):
        for x, _ in loader_train:
            if len(x) != batch_size:
                continue
            D_solver.zero_grad() # 判别器的梯度清零
            real_data = x.type(dtype)
            # 通过判别器D计算真实数据的分数logits_real
            logits_real = D(2* (real_data - 0.5)).type(dtype)

            # 生成一个噪声种子g_fake_seed，并通过生成器G生成假图像fake_images
            # 并将其从计算图中分离（detach）
            # 通过判别器D计算假图像的分数logits_fake
            g_fake_seed = sample_noise(batch_size, noise_size).type(dtype)
            fake_images = G(g_fake_seed).detach()
            logits_fake = D(fake_images.view(batch_size, 1, 28, 28))

            # D_solver进行参数更新
            d_total_error = discriminator_loss(logits_real, logits_fake)
            d_total_error.backward()
            D_solver.step()

            G_solver.zero_grad()
            g_fake_seed = sample_noise(batch_size, noise_size).type(dtype)
            fake_images = G(g_fake_seed)

            # 将生成器的梯度清零，并再次生成噪声种子g_fake_seed
            # 通过生成器G生成假图像fake_images。
            # 通过判别器D计算生成的假图像的分数gen_logits_fake。计算生成器的损失g_error
            gen_logits_fake = D(fake_images.view(batch_size, 1, 28, 28))
            g_error = generator_loss(gen_logits_fake)
            g_error.backward()
            # G_solver进行参数更新
            G_solver.step()

            if (iter_count % show_every == 0):
                print('Iter: {}, D: {:.4}, G:{:.4}'.format(iter_count,d_total_error.item(),g_error.item()))
                imgs_numpy = fake_images.data.cpu().numpy()
                images.append(imgs_numpy[0:16])

            iter_count += 1

    return images

# Least Squares GAN 最小二乘GAN

改变了损失函数

def ls_discriminator_loss(scores_real, scores_fake):
    """
    Compute the Least-Squares GAN loss for the discriminator.

    Inputs:
    - scores_real: PyTorch Tensor of shape (N,) giving scores for the real data.
    - scores_fake: PyTorch Tensor of shape (N,) giving scores for the fake data.

    Outputs:
    - loss: A PyTorch Tensor containing the loss.
    """
    loss = None
    # *****START OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****

    loss = 0.5 * ((scores_real-1)**2).mean(axis=0) + 0.5 * (scores_fake**2).mean(axis=0)

    # *****END OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****
    return loss

def ls_generator_loss(scores_fake):
    """
    Computes the Least-Squares GAN loss for the generator.

    Inputs:
    - scores_fake: PyTorch Tensor of shape (N,) giving scores for the fake data.

    Outputs:
    - loss: A PyTorch Tensor containing the loss.
    """
    loss = None
    # *****START OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****

    loss = 0.5 * ((scores_fake-1)**2).mean(axis=0)


    # *****END OF YOUR CODE (DO NOT DELETE/MODIFY THIS LINE)*****
    return loss

# Deeply Convolutional GANs

def build_dc_classifier(batch_size):
    """
    Build and return a PyTorch model for the DCGAN discriminator implementing
    the architecture above.
    """

    
    model = nn.Sequential(
      nn.Conv2d(in_channels=1, out_channels=32, kernel_size=5, stride=1, bias=True),
      nn.LeakyReLU(negative_slope=0.01),
      nn.MaxPool2d(kernel_size=2, stride=2),
      nn.Conv2d(in_channels=32, out_channels=64, kernel_size=5, stride=1, bias=True),
      nn.LeakyReLU(negative_slope=0.01),
      nn.MaxPool2d(kernel_size=2, stride=2),
      nn.Flatten(),
      nn.Linear(in_features=4*4*64, out_features=4*4*64, bias=True),
      nn.LeakyReLU(negative_slope=0.01),
      nn.Linear(in_features=4*4*64, out_features=1, bias=True),
      
    )

    return model


def build_dc_generator(noise_dim=NOISE_DIM):
    """
    Build and return a PyTorch model implementing the DCGAN generator using
    the architecture described above.
    """

    model = nn.Sequential(
      nn.Linear(in_features=noise_dim, out_features=1024, bias=True),
      nn.ReLU(),
      nn.BatchNorm1d(num_features=1024),
      nn.Linear(in_features=1024, out_features=7*7*128, bias=True),
      nn.ReLU(),
      nn.BatchNorm1d(num_features=7*7*128),
      nn.Unflatten(dim=1, unflattened_size=(128, 7, 7)),
      nn.ConvTranspose2d(in_channels=128, out_channels=64, kernel_size=4,
                         stride=2, padding=1),
      nn.ReLU(),
      nn.BatchNorm2d(num_features=64),
      nn.ConvTranspose2d(in_channels=64, out_channels=1, kernel_size=4,
                         stride=2, padding=1),
      nn.Tanh(),
      nn.Flatten(),
      
    )
    return model