pytorch dataset can't set attribute train_labels

first I set some paraments

import random
import torch
from torchvision import datasets

epochs = 50
lr = 1e-3
batch_size = 128
sample_size = 200
hidden_size = 200
num_task = 3

second I build an dataset,and def get_permute_mnist()

class PermutedMNIST(datasets.MNIST):

    def __init__(self, root="~/.torch/data/mnist", train=True, permute_idx=None):
        super(PermutedMNIST, self).__init__(root, train, download=True)
        assert len(permute_idx) == 28 * 28
        if self.train:
            self.train_data = torch.stack([img.float().view(-1)[permute_idx] / 255 for img in self.train_data])
        else:
            self.test_data = torch.stack([img.float().view(-1)[permute_idx] / 255 for img in self.test_data])

    def __getitem__(self, index):

        if self.train:
            img, target = self.train_data[index], self.train_labels[index]
        else:
            img, target = self.test_data[index], self.test_labels[index]

        return img, target

    def get_sample(self, sample_size):
        sample_idx = random.sample(range(len(self)), sample_size)
        return [img for img in self.train_data[sample_idx]]
def get_permute_mnist():
    train_loader = {}
    test_loader = {}
    idx = list(range(28 * 28))
    for i in range(num_task):
        train_loader[i] = torch.utils.data.DataLoader(PermutedMNIST(train=True, permute_idx=idx),
                                                      batch_size=batch_size,
                                                      num_workers=4)
        test_loader[i] = torch.utils.data.DataLoader(PermutedMNIST(train=False, permute_idx=idx),
                                                     batch_size=batch_size)
        random.shuffle(idx)
    return train_loader, test_loader

last I use the dataset

train_loader, test_loader = get_permute_mnist()

I get the following error AttributeError: can't set attribute, why I can set sel.train_data or give any value to the variable 'train_data'? the wrong is show in the picture: enter image description here