So each worker will essentially call your __getitem__ method batch_size number of times. Each time it’s getting an individual tensor. Afterwards, it will attempt to stack all of these tensors together to provide you a batch.
It appears here that two of those tensors were a mismatch. My best guess would be that the images in your dataset are not all the same sizes. Apparently one of them might be 25x20 and the other is 27x23. Using a resize and/or crop transformation should help here.