Thank you for your reply, @DAlolicorn.
I want to have a concatenation of test and train with the size of the test dataset for CIFAR-10. torch.utils.data.ConcatDataset will provide the dataset with the size of both datasets? I mean 50000+10000?
I also looking for the index of images. Is there any way, to get the index in  __getitem__  for each train and test dataset, not both of them one index?