Documentation states:
```cpp
// Shape of boxes input should be
// [batch_size, num_boxes, 4] or [batch_size, num_boxes, 1, 4] or [batch_size, num_boxes, num_classes, 4]
```
however, the second possibility `[batch_size, num_boxes, 1, 4]` triggers a false positive in an
assertion. The third dimension must be either equal to 1 or the number
of classes.
Signed-off-by: Stephan Seitz <sseitz@nvidia.com>