In the output it looks like one of the images is actually 3 channels instead of 4:
train:   0%| | 2/63647 [00:07<59:14:39,  3.35s/it, loss_mask: 1.0000, loss: 1.0000, mask_micro_iou: 0.0
torch.Size([1, 3, 768, 768])
['79771.tiff']