You could add the normalization in the __getitem__ function of your Dataset:
class MyDataset(Dataset):
    def __init__(self, X, y, transform=None):
        self.data = X
        self.target = y
        self.transform = transform
        
    def __getitem__(self, index):
        x = self.data[index]
        y = self.target[index]

        # Normalize your data here
        if self.transform:
            x = self.transform(x)

        return x, y
    
    def __len__(self):
        return len(self.data)

In this use case, you could set transform to something like this:
transforms.Compose([
    transforms.ToTensor(),
    transforms.Normalize(mean=[0.485, 0.456, 0.406],
                         std=[0.229, 0.224, 0.225])
])