pytorch框架的代码问题。

import os
import pandas as pd
from torchvision.io import read_image

class CustomImageDataset(Dataset):
def init(self, annotations_file, img_dir, transform=None, target_transform=None):
self.img_labels = pd.read_csv(annotations_file)
self.img_dir = img_dir
self.transform = transform
self.target_transform = target_transform

def __len__(self):
    return len(self.img_labels)

def __getitem__(self, idx):
    img_path = os.path.join(self.img_dir, self.img_labels.iloc[idx, 0])
    image = read_image(img_path)
    label = self.img_labels.iloc[idx, 1]
    if self.transform:
        image = self.transform(image)
    if self.target_transform:
        label = self.target_transform(label)
    return image, label

请问下面的这两行代码竟起到
image = self.transform(image)
label = self.target_transform(label)
了什么样的作用？