首发：AI公园公众号
作者：Marek Paulik
编译：ronghuaiyang
导读
一个非常简单和容易上手的例子。
对于教程中使用的大多数人工数据集，每个类都有相同数量的数据。然而，在实际应用中，这种情况很少发生。今天，我将给你介绍来自Kaggle的木薯叶分类，并告诉你当类频率有很大差异时该怎么做。
处理类别的不平衡
有两种方法可以解决这个问题。
- WeightedRandomSampler
- loss函数中的weight参数
下一步是创建一个有5个方法的CassavaClassifier类：load\_data()、load\_model()、fit\_one\_epoch()、val\_one\_epoch()和fit()。
在load\_data()中，将构造一个train和验证数据集，并返回数据加载器以供进一步使用。
在load\_model()中定义了体系结构、损失函数和优化器。
fit方法包含一些初始化和对fit\_one\_epoch()和val\_one\_epoch()的循环。
早期停止
早期停止类有助于根据验证损失跟踪最佳模型，并保存检查点。
#Callbacks# Early stoppingclass EarlyStopping: def __init__(self, patience=1, delta=0, path='checkpoint.pt'): self.patience = patience self.delta = delta self.path= path self.counter = 0 self.best_score = None self.early_stop = False def __call__(self, val_loss, model): if self.best_score is None: self.best_score = val_loss self.save_checkpoint(model) elif val_loss > self.best_score: self.counter +=1 if self.counter >= self.patience: self.early_stop = True else: self.best_score = val_loss self.save_checkpoint(model) self.counter = 0 def save_checkpoint(self, model): torch.save(model.state_dict(), self.path)Init
我们首先初始化CassavaClassifier类。
class CassavaClassifier(): def __init__(self, data_dir, num_classes, device, Transform=None, sample=False, loss_weights=False, batch_size=16, lr=1e-4, stop_early=True, freeze_backbone=True): ############################################################################################################# # data_dir - directory with images in subfolders, subfolders name are categories # Transform - data augmentations # sample - if the dataset is imbalanced set to true and RandomWeightedSampler will be used # loss_weights - if the dataset is imbalanced set to true and weight parameter will be passed to loss function # freeze_backbone - if using pretrained architecture freeze all but the classification layer ############################################################################################################### self.data_dir = data_dir self.num_classes = num_classes self.device = device self.sample = sample self.loss_weights = loss_weights self.batch_size = batch_size self.lr = lr self.stop_early = stop_early self.freeze_backbone = freeze_backbone self.Transform = TransformLoad Data
训练图像被组织在子文件夹中，子文件夹名称表示图像的类。这是图像分类问题的典型情况，幸运的是，不需要编写自定义数据集类。在这种情况下，可以立即使用torchvision中的ImageFolder。如果你想使用WeightedRandomSampler，你需要为数据集的每个元素指定一个权重。通常，总图像总比上类别数被用作一个权重。
def load_data(self): train_full = torchvision.datasets.ImageFolder(self.data_dir, transform=self.Transform) train_set, val_set = random_split(train_full, [math.floor(len(train_full)*0.8), math.ceil(len(train_full)*0.2)]) self.train_classes = [label for _, label in train_set] if self.sample: # Need to get weight for every image in the dataset class_count = Counter(self.train_classes) class_weights = torch.Tensor([len(self.train_classes)/c for c in pd.Series(class_count).sort_index().values]) # Can't iterate over class_count because dictionary is unordered sample_weights = [0] * len(train_set) for idx, (image, label) in enumerate(train_set): class_weight = class_weights[label] sample_weights[idx] = class_weight sampler = WeightedRandomSampler(weights=sample_weights, num_samples = len(train_set), replacement=True) train_loader = DataLoader(train_set, batch_size=self.batch_size, sampler=sampler) else: train_loader = DataLoader(train_set, batch_size=self.batch_size, shuffle=True) val_loader = DataLoader(val_set, batch_size=self.batch_size) return train_loader, val_loaderLoad Model
在该方法中，我使用迁移学习，架构参数从预先训练的resnet50和efficientnet-b7中选择。CrossEntropyLoss和许多其他损失函数都有权重参数。这是一个手动调整参数，用于处理不平衡。在这种情况下，不需要为每个参数定义权重，只需为每个类定义权重。
def load_model(self, arch='resnet'): ############################################################################################################## # arch - choose the pretrained architecture from resnet or efficientnetb7 ############################################################################################################## if arch == 'resnet': self.model = torchvision.models.resnet50(pretrained=True) if self.freeze_backbone: for param in self.model.parameters(): param.requires_grad = False self.model.fc = nn.Linear(in_features=self.model.fc.in_features, out_features=self.num_classes) elif arch == 'efficient-net': self.model = EfficientNet.from_pretrained('efficientnet-b7') if self.freeze_backbone: for param in self.model.parameters(): param.requires_grad = False self.model._fc = nn.Linear(in_features=self.model._fc.in_features, out_features=self.num_classes) self.model = self.model.to(self.device) self.optimizer = torch.optim.Adam(self.model.parameters(), self.lr) if self.loss_weights: class_count = Counter(self.train_classes) class_weights = torch.Tensor([len(self.train_classes)/c for c in pd.Series(class_count).sort_index().values]) # Cant iterate over class_count because dictionary is unordered class_weights = class_weights.to(self.device) self.criterion = nn.CrossEntropyLoss(class_weights) else: self.criterion = nn.CrossEntropyLoss() Fit One Epoch
这个方法只包含一个经典的训练循环，带有训练损失记录和tqdm进度条。
def fit_one_epoch(self, train_loader, epoch, num_epochs ): step_train = 0 train_losses = list() # Every epoch check average loss per batch train_acc = list() self.model.train() for i, (images, targets) in enumerate(tqdm(train_loader)): images = images.to(self.device) targets = targets.to(self.device) logits = self.model(images) loss = self.criterion(logits, targets) loss.backward() self.optimizer.step() self.optimizer.zero_grad() train_losses.append(loss.item()) #Calculate running train accuracy predictions = torch.argmax(logits, dim=1) num_correct = sum(predictions.eq(targets)) running_train_acc = float(num_correct) / float(images.shape[0]) train_acc.append(running_train_acc) train_loss = torch.tensor(train_losses).mean() print(f'Epoch {epoch}/{num_epochs-1}') print(f'Training loss: {train_loss:.2f}')Validate one epoch
与上面类似，但此方法在验证数据加载器上迭代。在每一个epoch'之后，平均batch损失和准确性被打印出来。
def val_one_epoch(self, val_loader, scaler): val_losses = list() val_accs = list() self.model.eval() step_val = 0 with torch.no_grad(): for (images, targets) in val_loader: images = images.to(self.device) targets = targets.to(self.device) logits = self.model(images) loss = self.criterion(logits, targets) val_losses.append(loss.item()) predictions = torch.argmax(logits, dim=1) num_correct = sum(predictions.eq(targets)) running_val_acc = float(num_correct) / float(images.shape[0]) val_accs.append(running_val_acc) self.val_loss = torch.tensor(val_losses).mean() val_acc = torch.tensor(val_accs).mean() # Average acc per batch print(f'Validation loss: {self.val_loss:.2f}') print(f'Validation accuracy: {val_acc:.2f}') Fit
Fit方法在训练和验证过程中经历了许多阶段和循环。如果预训练模型的参数在开始时被冻结，那么unfreeze\_after定义了整个模型在多少个epoch之后开始训练。在此之前，只训练全连接层(分类器)。
def fit(self, train_loader, val_loader, num_epochs=10, unfreeze_after=5, checkpoint_dir='checkpoint.pt'): if self.stop_early: early_stopping = EarlyStopping( patience=5, path=checkpoint_dir) for epoch in range(num_epochs): if self.freeze_backbone: if epoch == unfreeze_after: # Unfreeze after x epochs for param in self.model.parameters(): param.requires_grad = True self.fit_one_epoch(train_loader, scaler, epoch, num_epochs) self.val_one_epoch(val_loader, scaler) if self.stop_early: early_stopping(self.val_loss, self.model) if early_stopping.early_stop: print('Early Stopping') print(f'Best validation loss: {early_stopping.best_score}') breakRun
现在，可以初始化CassavaClassifier类、创建dataloaders、设置模型并运行整个过程了。
Transform = T.Compose( [T.ToTensor(), T.Resize((256, 256)), T.RandomRotation(90), T.RandomHorizontalFlip(p=0.5), T.Normalize((0.485, 0.456, 0.406), (0.229, 0.224, 0.225))])device = torch.device('cuda') if torch.cuda.is_available() else torch.device('cpu')data_dir = "Data/cassava-disease/train/train"classifier = CassavaClassifier(data_dir=data_dir, num_classes=5, device=device, sample=True, Transform=Transform)train_loader, val_loader = classifier.load_data()classifier.load_model()classifier.fit(num_epochs=20, unfreeze_after=5, train_loader=train_loader, val_loader=val_loader)Inference
使用ImageFolder加载测试数据是不可能的，因为显然没有带有类的子文件夹。因此，我创建了一个返回图像和图像id的自定义数据集。随后，加载模型检查点，通过推理循环运行它，并将预测保存到数据帧中。将数据帧导出为CSV并提交结果。
# Inferencemodel = torchvision.models.resnet50()#model = EfficientNet.from_name('efficientnet-b7')model.fc = nn.Linear(in_features=model.fc.in_features, out_features=5)model = model.to(device)checkpoint = torch.load('Data/cassava-disease/sampler_checkpoint.pt')model.load_state_dict(checkpoint)model.eval()# Dataset for test dataclass Cassava_Test(Dataset): def __init__(self, dir, transform=None): self.dir = dir self.transform = transform self.images = os.listdir(self.dir) def __len__(self): return len(self.images) def __getitem__(self, idx): img = Image.open(os.path.join(self.dir, self.images[idx])) return self.transform(img), self.images[idx] test_dir = 'Data/cassava-disease/test/test/0'test_set = Cassava_Test(test_dir, transform=Transform)test_loader = DataLoader(test_set, batch_size=4) # Test loopsub = pd.DataFrame(columns=['category', 'id'])id_list = []pred_list = []model = model.to(device)with torch.no_grad(): for (image, image_id) in test_loader: image = image.to(device) logits = model(image) predicted = list(torch.argmax(logits, 1).cpu().numpy()) for id in image_id: id_list.append(id) for prediction in predicted: pred_list.append(prediction)sub['category'] = pred_listsub['id'] = id_listmapping = {0:'cbb', 1:'cbsd', 2:'cgm', 3:'cmd', 4:'healthy'}sub['category'] = sub['category'].map(mapping)sub = sub.sort_values(by='id')sub.to_csv('Cassava_sub.csv', index=False)如果在方案中包含WeightedRandomSampler或损失权值，则测试集的精度会提高2%。对于仅仅几行代码来说，这是一个很好的改进。对于这个数据集，我没有看到这两种方法在精度上的巨大差异，但WeightedRandomSampler的表现要好一些。
不同的学习速度、优化器和数据扩展肯定有自己的发展空间。然而，对于这种简单的方法来说，86%的准确率似乎足够好了。
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英文原文：https://marekpaulik.medium.co...
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