迁移学习（五）：知识蒸馏

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import torch
import torch.nn as nn
import torch.nn.functional as F
import torch.optim as optim
from torch.utils.data import DataLoader
import torchvision
import torchvision.transforms as transforms
from typing import List
import copy


# ===== 损失函数 =====

class KDLoss(nn.Module):
    """响应蒸馏：带温度的 KL 散度 + 硬标签 CE。"""
    def __init__(self, temperature: float = 4.0, alpha: float = 0.9):
        super().__init__()
        self.T = temperature
        self.alpha = alpha

    def forward(self, student_logits, teacher_logits, labels):
        kd = F.kl_div(
            F.log_softmax(student_logits / self.T, dim=1),
            F.softmax(teacher_logits / self.T, dim=1),
            reduction='batchmean',
        ) * (self.T ** 2)
        ce = F.cross_entropy(student_logits, labels)
        return self.alpha * kd + (1 - self.alpha) * ce


class FeatureDistillLoss(nn.Module):
    """FitNets：1x1 投影后做特征图 MSE。"""
    def __init__(self, student_ch: int, teacher_ch: int):
        super().__init__()
        self.proj = nn.Conv2d(student_ch, teacher_ch, 1, bias=False)

    def forward(self, student_feat, teacher_feat):
        return F.mse_loss(self.proj(student_feat), teacher_feat)


class AttentionTransferLoss(nn.Module):
    """对通道做 L^p 聚合并归一化的空间注意力图匹配。"""
    def __init__(self, p: float = 2.0):
        super().__init__()
        self.p = p

    def _attn(self, feat):
        a = torch.sum(torch.abs(feat) ** self.p, dim=1, keepdim=True)
        return a / (a.sum(dim=[2, 3], keepdim=True) + 1e-8)

    def forward(self, s_feat, t_feat):
        return F.mse_loss(self._attn(s_feat), self._attn(t_feat))


class RelationalLoss(nn.Module):
    """RKD：成对距离 + 余弦（角度）关系。"""
    def __init__(self, w_dist: float = 1.0, w_angle: float = 2.0):
        super().__init__()
        self.w_dist, self.w_angle = w_dist, w_angle

    def forward(self, s_feat, t_feat):
        s = F.normalize(s_feat, p=2, dim=1)
        t = F.normalize(t_feat, p=2, dim=1)
        d_loss = F.mse_loss(torch.cdist(s, s), torch.cdist(t, t))
        a_loss = F.mse_loss(s @ s.t(), t @ t.t())
        return self.w_dist * d_loss + self.w_angle * a_loss


# ===== 模型（同时返回 logits 与中间特征）=====

class _ResNetBackbone(nn.Module):
    def __init__(self, ctor, num_classes: int):
        super().__init__()
        self.net = ctor(weights=None)
        self.net.fc = nn.Linear(self.net.fc.in_features, num_classes)

    def forward(self, x):
        n = self.net
        x = n.maxpool(n.relu(n.bn1(n.conv1(x))))
        feats: List[torch.Tensor] = []
        for layer in (n.layer1, n.layer2, n.layer3, n.layer4):
            x = layer(x)
            feats.append(x)
        logits = n.fc(torch.flatten(n.avgpool(x), 1))
        return logits, feats


class TeacherNet(_ResNetBackbone):
    def __init__(self, num_classes: int = 10):
        super().__init__(torchvision.models.resnet34, num_classes)


class StudentNet(_ResNetBackbone):
    def __init__(self, num_classes: int = 10):
        super().__init__(torchvision.models.resnet18, num_classes)


# ===== 训练器（response / feature / attention / relation / combined）=====

class DistillationTrainer:
    def __init__(self, teacher, student, device='cpu',
                 mode: str = 'response',
                 temperature: float = 4.0, alpha: float = 0.9):
        self.teacher = teacher.to(device).eval()
        self.student = student.to(device)
        self.device = device
        self.mode = mode
        for p in self.teacher.parameters():
            p.requires_grad = False

        self.kd = KDLoss(temperature, alpha)
        if mode in ('feature', 'combined'):
            ch = [64, 128, 256, 512]
            self.feat = nn.ModuleList(
                [FeatureDistillLoss(s, t).to(device)
                 for s, t in zip(ch, ch)])
        if mode in ('attention', 'combined'):
            self.attn = AttentionTransferLoss()
        if mode in ('relation', 'combined'):
            self.rel = RelationalLoss()

    def _loss(self, s_logits, s_feats, t_logits, t_feats, y):
        loss = self.kd(s_logits, t_logits, y)
        if self.mode in ('feature', 'combined'):
            fl = sum(fn(sf, tf)
                     for fn, sf, tf in zip(self.feat, s_feats, t_feats))
            loss = loss + 0.5 * fl / len(s_feats)
        if self.mode in ('attention', 'combined'):
            al = sum(self.attn(sf, tf)
                     for sf, tf in zip(s_feats, t_feats))
            loss = loss + 0.3 * al / len(s_feats)
        if self.mode in ('relation', 'combined'):
            loss = loss + 0.2 * self.rel(
                s_feats[-1].flatten(1), t_feats[-1].flatten(1))
        return loss

    def train_epoch(self, loader, optimizer):
        self.student.train()
        total, correct, n = 0.0, 0, 0
        for x, y in loader:
            x, y = x.to(self.device), y.to(self.device)
            with torch.no_grad():
                t_logits, t_feats = self.teacher(x)
            s_logits, s_feats = self.student(x)
            loss = self._loss(s_logits, s_feats, t_logits, t_feats, y)
            optimizer.zero_grad(); loss.backward(); optimizer.step()
            total += loss.item() * y.size(0)
            correct += (s_logits.argmax(1) == y).sum().item()
            n += y.size(0)
        return total / n, 100.0 * correct / n

    @torch.no_grad()
    def evaluate(self, loader):
        self.student.eval()
        correct, n = 0, 0
        for x, y in loader:
            x, y = x.to(self.device), y.to(self.device)
            logits, _ = self.student(x)
            correct += (logits.argmax(1) == y).sum().item()
            n += y.size(0)
        return 100.0 * correct / n


# ===== 自蒸馏：Born-Again Networks =====

def self_distill(model_class, train_loader, test_loader,
                 num_generations: int = 3, epochs_per_gen: int = 10,
                 device: str = 'cpu', temperature: float = 4.0):
    teacher = None
    for gen in range(num_generations):
        student = model_class().to(device)
        opt = optim.SGD(student.parameters(), lr=0.1,
                        momentum=0.9, weight_decay=5e-4)
        sched = optim.lr_scheduler.CosineAnnealingLR(opt, epochs_per_gen)

        for _ in range(epochs_per_gen):
            student.train()
            for x, y in train_loader:
                x, y = x.to(device), y.to(device)
                logits, _ = student(x)
                loss = F.cross_entropy(logits, y)
                if teacher is not None:
                    with torch.no_grad():
                        t_logits, _ = teacher(x)
                    kd = F.kl_div(
                        F.log_softmax(logits / temperature, dim=1),
                        F.softmax(t_logits / temperature, dim=1),
                        reduction='batchmean') * temperature ** 2
                    loss = 0.1 * loss + 0.9 * kd
                opt.zero_grad(); loss.backward(); opt.step()
            sched.step()

        student.eval()
        correct, n = 0, 0
        with torch.no_grad():
            for x, y in test_loader:
                x, y = x.to(device), y.to(device)
                correct += (student(x)[0].argmax(1) == y).sum().item()
                n += y.size(0)
        print(f"Generation {gen + 1}: {100.0 * correct / n:.2f}%")

        teacher = copy.deepcopy(student).eval()
        for p in teacher.parameters():
            p.requires_grad = False


# ===== 主流程 =====

def main():
    device = 'cuda' if torch.cuda.is_available() else 'cpu'
    norm = transforms.Normalize((0.4914, 0.4822, 0.4465),
                                (0.2023, 0.1994, 0.2010))
    train_tf = transforms.Compose([
        transforms.RandomCrop(32, padding=4),
        transforms.RandomHorizontalFlip(),
        transforms.ToTensor(), norm])
    test_tf = transforms.Compose([transforms.ToTensor(), norm])

    train_set = torchvision.datasets.CIFAR10(
        './data', train=True, download=True, transform=train_tf)
    test_set = torchvision.datasets.CIFAR10(
        './data', train=False, download=True, transform=test_tf)
    train_loader = DataLoader(train_set, 128, shuffle=True, num_workers=2)
    test_loader = DataLoader(test_set, 128, num_workers=2)

    teacher = TeacherNet(10)
    student = StudentNet(10)
    trainer = DistillationTrainer(
        teacher, student, device, mode='combined',
        temperature=4.0, alpha=0.7)
    opt = optim.SGD(student.parameters(), lr=0.1,
                    momentum=0.9, weight_decay=5e-4)
    for epoch in range(20):
        loss, acc = trainer.train_epoch(train_loader, opt)
        test_acc = trainer.evaluate(test_loader)
        print(f"Epoch {epoch + 1}: loss={loss:.4f} "
              f"train={acc:.1f}% test={test_acc:.1f}%")

    print("\n自蒸馏（Born-Again Networks）：")
    self_distill(StudentNet, train_loader, test_loader,
                 num_generations=3, epochs_per_gen=10, device=device)


if __name__ == '__main__':
    main()