random_crop results in image with zero target.

[chenyuntc]

random_crop op may crop only background part, which will result in the cls_targets to all be zero. This will raise ZeroDivisionError: float division by zero in here when all item in the batch doesn't include background (num_pos is 0)

[vaishnavm217]

I am facing this same problem. for now, I have given num_pos very less value, but all my losses are coming zero.

[ahkarami]

@vaishnavm217 & @chenyuntc.
Please refer to #35