Create ImageFolder with the same (sorted) order on different machines

[JirenJin]

As mentioned in #192, current make_dataset function for ImageFolder may create datasets with different orders because os.listdir(dir) might produce different results on different machines.

This can be inconvenient if the user wants to compare or ensemble the outputs of the full dataset (for example, the validation set).

A simple modification for target in sorted(os.listdir(dir)): solves the problem. And since the loading procedure itself can be shuffled, I have not found the possible problems can be caused by this modification.

[JirenJin]

Also, it is difficult to reproduce the results on different machines. Even setting random seed will not produce the same ordered shuffle in training because the original orders of data are different.

[soumith]

thanks!