why the 2nd element of the yield in data_generator is of size equal to batch_size?

[sonalrpatel]

Following is the link to the exact line of code.
I just want to understand the theory behind it.

keras-yolo3/train.py

Line 182 in e6598d1

 yield [image_data, *y_true], np.zeros(batch_size) 

[sonalrpatel]

Also refer to following link for another similar question.
#761 (comment)