About the variable of "gt_bo_masks"

I want to train BCNet using my customized dataset, which is made according to the instructions in the Mask2former project. However, I am encountering an error that says, ''AttributeError: Cannot find field 'gt_bo_masks' in the given Instances!''
I haven't found any definition of this variable in official documents. So what does it mean in the BCNet project? 