prior_box size is not ok

[Ontheway361]

In my view, the vision of SSD512 is not the ok, becase thare are some errors in size between prior_box and loc, conf. Maybe you miss the layers in multibox ?

[Ontheway361]

Old iron， attention：
extras = {
'300': [256, 'S', 512, 128, 'S', 256, 128, 256, 128, 256],
'512': [256, 'S', 512, 128, 'S', 256, 128, 'S', 256, 128, 'S', 256, 128, 'S', 256],
}