Vertical flip?

[LaurentBerder]

In train_utils.py, you've made a horizontal flip:

` # 5. Random horizontal flipping

    img, params = transforms.random_flip(
        img, x_random=True, return_param=True)

    bbox = transforms.flip_bbox(
        bbox, (self.size, self.size), x_flip=params['x_flip'])

`

I'm wondering if there is a reason why you didn't also implement a vertical flip (with a #6 paragraph, copy of #5 with x_random replaced by y_random, and x_flip by y_flip), which would again double the chances of recognizing objects.