do not know the purpose of the following codes in nets/yolo_v2.py

[Follow_your_heart]

object_mask = tf.reduce_sum(gbboxes_batch, 4)
object_mask = tf.cast(object_mask > 0, tf.float32)

i do not know what is the object_mask what the purpose of the two operations. what's more, the shape of the gbboxes_batch is ...?