I want to concat the PASCAL VOC 2007 train dataset and PASCAL VOC 2007 val dataset, but COCO only offer two individual JSON for them. I have to create two dataset instances for them and concate them.

However, itertools.chain does not work while it does not have __len__ method. So I have to use a wrapper class to concate them.

If dataset class support __add__, then it is so convenient that we can use trainval_dataset = train_dataset + val_dataset immediately.

If you agree, I will make the pull request later.