What you are loading is the train_loader.  You can get a batch of images from it using
images, labels = next(iter(train_loader))

The shapes of these are as follows
images.shape
> (torch.Size([128, 1, 28, 28]))

For plotting you need to use the last 2 dimensions (width and height)
plt.imshow(images[0].reshape(28,28), cmap="gray")