Bugfix for single loader case

[m-parchami]

Hi,

Thanks for the clean repo. I ran a sanity check with giving the exact replica of the same model, expecting to get an identity diagonal for the output of plot_results(), but it wasn't the case.

After looking at the code, I noticed that there is a line that the case that dataloader2 is not given has some issues. As it creates a reference copy of dataloader1 and hence, when zipped together in the loop, same loader is being used for getting x1 and x2, so they would be different indices of the dataset, resulting in different samples.

Looking forward to hearing from you! Best.