I believe if one side feels sincere regret and active measures are taken to make sure that the atrocities can never occur again then over time atrocities can be forgiven but never forgotten, it is my belief that very little good comes out of holding a grudge and just brings up more problems down the line.
Forgiven but not forgotten might be the best way to put it. People such as Nelson Mandela show us that an open heart is not unheard of when it comes to dealing with terrible things some people have done to you, so it is possible but it takes a big soul to accomplish it.
Yes, atrocities can ever be forgiven, because there is always a chance that the person can turn around and apologize for what they have done. That does not mean that it is likely. In order to truly forgive, the person has to understand how terrible their actions were. Although this is not likely, it is possible.
I believe you can't just flat out and forgive someone. Pain you are suffering might fade over time but for now you are stuck with It. Also, you will probably never forget an atrocity because it will Invade your mind from time to time, but with time you can decide depending on the situation you can forgive someone
There is no such thing as forgiveness, there are only short memories. There is no reason an atrocity like mass genocide or rape should ever be forgiven or forgotten, and it should stain generations and generations of people to come, because we need to understand and accept our sordid histories as people.
I do not think atrocities can ever be ever forgiven. But with time, the pain becomes easier to bear. I think that is why some people are able to forgive them. But like I said, there will never be a way to forgive and forget. At least that's what I think.