Change '__getitem__' function in DepthDataset class

[Taeyoung96]

Hi! I'm using your great code.
I have a suggestion about __getitem__ function in DepthDataset class in UtilityTest.py.

I just execute the test_img.ipynb and I got a lot of depth prediction images.
In my case, It is important to get ordered data to match with input RGB images.
However, when I use these images, the order is different from the input images!

So, I just add sorted() function, to make the output in the same order as the input image list.

I think it is more helpful when you just add this function!

Thanks.