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预训练模型

1. 预训练模型的概念

我们之前在训练中发现，准确率最开始随着epoch的增加而增加。随着循环的更新，参数在不断发生更新。

所以参数的初始值对训练结果有很大的影响：

1. 如果最开始的初始值比较好，后续训练轮数就会少很多
2. 很有可能陷入局部最优值，不同的初始值可能导致陷入不同的局部最优值

所以很自然的想到，如果最开始能有比较好的参数，即可能导致未来训练次数少，也可能导致未来训练避免陷入局部最优解的问题。这就引入了一个概念，即预训练模型

2. 预训练模型的策略

那么什么模型会被选为预训练模型呢？比如一些调参后表现很好的cnn神经网络（固定的神经元个数+固定的层数等）。

所以调用预训练模型做微调，本质就是 用这些固定的结构+之前训练好的参数接着训练，所以需要找到预训练的模型结构并且加载模型参数

相较于之前用自己定义的模型有以下几个注意点

1. 需要调用预训练模型和加载权重
2. 需要resize 图片让其可以适配模型
3. 需要修改最后的全连接层以适应数据集

其中，训练过程中，为了不破坏最开始的特征提取器的参数，最开始往往先冻结住特征提取器的参数，然后训练全连接层，大约在5-10个epoch后解冻训练

主要做特征提取的部分叫做backbone骨干网络；负责融合提取的特征的部分叫做Featue Pyramid Network（FPN）；负责输出的预测部分的叫做Head

import torch
import torch.nn as nn
import torch.optim as optim
from torchvision import datasets, transforms, models
from torch.utils.data import DataLoader
import matplotlib.pyplot as plt
import os# 设置中文字体支持
plt.rcParams["font.family"] = ["SimHei"]
plt.rcParams['axes.unicode_minus'] = False  # 解决负号显示问题import torch
import torch.nn as nn
import torch.optim as optim
from torchvision import datasets, transforms, models
from torch.utils.data import DataLoader
import matplotlib.pyplot as plt
import os# 设置中文字体支持
plt.rcParams["font.family"] = ["SimHei"]
plt.rcParams['axes.unicode_minus'] = False  # 解决负号显示问题# 检查GPU是否可用
device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
print(f"使用设备: {device}")# 1. 数据预处理（训练集增强，测试集标准化）
train_transform = transforms.Compose([transforms.RandomCrop(32, padding=4),  # 随机裁剪transforms.RandomHorizontalFlip(),  # 随机水平翻转transforms.ColorJitter(brightness=0.2, contrast=0.2, saturation=0.2, hue=0.1),  # 颜色抖动transforms.RandomRotation(15),  # 随机旋转transforms.ToTensor(),  # 转换为张量transforms.Normalize((0.4914, 0.4822, 0.4465), (0.2023, 0.1994, 0.2010))  # 标准化
])
# 测试集只需要标准化
test_transform = transforms.Compose([transforms.ToTensor(),transforms.Normalize((0.4914, 0.4822, 0.4465), (0.2023, 0.1994, 0.2010))
])# 2. 加载CIFAR-10数据集
train_dataset = datasets.CIFAR10(root='./data',train=True,download=True,transform=train_transform
)test_dataset = datasets.CIFAR10(root='./data',train=False,transform=test_transform
)# 3. 创建数据加载器
batch_size = 64
train_loader = DataLoader(train_dataset, batch_size=batch_size, shuffle=True)
test_loader = DataLoader(test_dataset, batch_size=batch_size, shuffle=False)# 4. 定义ResNet18模型
def create_resnet18(pretrained=True, num_classes=10):model = models.resnet18(pretrained=pretrained)# 修改最后一层全连接层in_features = model.fc.in_featuresmodel.fc = nn.Linear(in_features, num_classes)return model.to(device)# 5. 冻结/解冻模型层的函数
def freeze_model(model, freeze=True):"""冻结或解冻模型的卷积层参数"""# 冻结/解冻除fc层外的所有参数for name, param in model.named_parameters():if 'fc' not in name:param.requires_grad = not freeze# 打印冻结状态frozen_params = sum(p.numel() for p in model.parameters() if not p.requires_grad)total_params = sum(p.numel() for p in model.parameters())if freeze:print(f"已冻结模型卷积层参数 ({frozen_params}/{total_params} 参数)")else:print(f"已解冻模型所有参数 ({total_params}/{total_params} 参数可训练)")return model# 6. 训练函数（支持阶段式训练）
def train_with_freeze_schedule(model, train_loader, test_loader, criterion, optimizer, scheduler, device, epochs, freeze_epochs=5):"""前freeze_epochs轮冻结卷积层，之后解冻所有层进行训练"""train_loss_history = []test_loss_history = []train_acc_history = []test_acc_history = []all_iter_losses = []iter_indices = []# 初始冻结卷积层if freeze_epochs > 0:model = freeze_model(model, freeze=True)for epoch in range(epochs):# 解冻控制：在指定轮次后解冻所有层if epoch == freeze_epochs:model = freeze_model(model, freeze=False)# 解冻后调整优化器（可选）optimizer.param_groups[0]['lr'] = 1e-4  # 降低学习率防止过拟合model.train()  # 设置为训练模式running_loss = 0.0correct_train = 0total_train = 0for batch_idx, (data, target) in enumerate(train_loader):data, target = data.to(device), target.to(device)optimizer.zero_grad()output = model(data)loss = criterion(output, target)loss.backward()optimizer.step()# 记录Iteration损失iter_loss = loss.item()all_iter_losses.append(iter_loss)iter_indices.append(epoch * len(train_loader) + batch_idx + 1)# 统计训练指标running_loss += iter_loss_, predicted = output.max(1)total_train += target.size(0)correct_train += predicted.eq(target).sum().item()# 每100批次打印进度if (batch_idx + 1) % 100 == 0:print(f"Epoch {epoch+1}/{epochs} | Batch {batch_idx+1}/{len(train_loader)} "f"| 单Batch损失: {iter_loss:.4f}")# 计算 epoch 级指标epoch_train_loss = running_loss / len(train_loader)epoch_train_acc = 100. * correct_train / total_train# 测试阶段model.eval()correct_test = 0total_test = 0test_loss = 0.0with torch.no_grad():for data, target in test_loader:data, target = data.to(device), target.to(device)output = model(data)test_loss += criterion(output, target).item()_, predicted = output.max(1)total_test += target.size(0)correct_test += predicted.eq(target).sum().item()epoch_test_loss = test_loss / len(test_loader)epoch_test_acc = 100. * correct_test / total_test# 记录历史数据train_loss_history.append(epoch_train_loss)test_loss_history.append(epoch_test_loss)train_acc_history.append(epoch_train_acc)test_acc_history.append(epoch_test_acc)# 更新学习率调度器if scheduler is not None:scheduler.step(epoch_test_loss)# 打印 epoch 结果print(f"Epoch {epoch+1} 完成 | 训练损失: {epoch_train_loss:.4f} "f"| 训练准确率: {epoch_train_acc:.2f}% | 测试准确率: {epoch_test_acc:.2f}%")# 绘制损失和准确率曲线plot_iter_losses(all_iter_losses, iter_indices)plot_epoch_metrics(train_acc_history, test_acc_history, train_loss_history, test_loss_history)return epoch_test_acc  # 返回最终测试准确率# 7. 绘制Iteration损失曲线
def plot_iter_losses(losses, indices):plt.figure(figsize=(10, 4))plt.plot(indices, losses, 'b-', alpha=0.7)plt.xlabel('Iteration（Batch序号）')plt.ylabel('损失值')plt.title('训练过程中的Iteration损失变化')plt.grid(True)plt.show()# 8. 绘制Epoch级指标曲线
def plot_epoch_metrics(train_acc, test_acc, train_loss, test_loss):epochs = range(1, len(train_acc) + 1)plt.figure(figsize=(12, 5))# 准确率曲线plt.subplot(1, 2, 1)plt.plot(epochs, train_acc, 'b-', label='训练准确率')plt.plot(epochs, test_acc, 'r-', label='测试准确率')plt.xlabel('Epoch')plt.ylabel('准确率 (%)')plt.title('准确率随Epoch变化')plt.legend()plt.grid(True)# 损失曲线plt.subplot(1, 2, 2)plt.plot(epochs, train_loss, 'b-', label='训练损失')plt.plot(epochs, test_loss, 'r-', label='测试损失')plt.xlabel('Epoch')plt.ylabel('损失值')plt.title('损失值随Epoch变化')plt.legend()plt.grid(True)plt.tight_layout()plt.show()# 主函数：训练模型
def main():# 参数设置epochs = 40  # 总训练轮次freeze_epochs = 5  # 冻结卷积层的轮次learning_rate = 1e-3  # 初始学习率weight_decay = 1e-4  # 权重衰减# 创建ResNet18模型（加载预训练权重）model = create_resnet18(pretrained=True, num_classes=10)# 定义优化器和损失函数optimizer = optim.Adam(model.parameters(), lr=learning_rate, weight_decay=weight_decay)criterion = nn.CrossEntropyLoss()# 定义学习率调度器scheduler = optim.lr_scheduler.ReduceLROnPlateau(optimizer, mode='min', factor=0.5, patience=2, verbose=True)# 开始训练（前5轮冻结卷积层，之后解冻）final_accuracy = train_with_freeze_schedule(model=model,train_loader=train_loader,test_loader=test_loader,criterion=criterion,optimizer=optimizer,scheduler=scheduler,device=device,epochs=epochs,freeze_epochs=freeze_epochs)print(f"训练完成！最终测试准确率: {final_accuracy:.2f}%")if __name__ == "__main__":main()