iou computation

I have found in the train.py the iou function
`start_i, end_i = interval[0], interval[1]`

`intersection = max(0, min(end, end_i) - max(start, start_i))`
`union = min(max(end, end_i) - min(start, start_i), end - start + end_i - start_i)`
`overlap = float(intersection) / (union + 1e-8)`

when the start time is negative, whether the iou should be 0 anyway?