Add skip connnection to resnet backbone

[menggui1993]

@xingyizhou Hi, CenterNet is really a great work. I came across the question when I was checking the detail of net structure. In Resnet backbone Centernet, there's no skip connection. I think it's a quite common trick to concat downsampling features to upsampling features, just like in dla34 or hourglass. A simple way is like this, Have you ever tried to add skip connection in Centernet-resnet? Is there any reason you didn't add skip connection to Centernet-resnet? Thanks.

[niecongchong]

it‘s right. The center's location accuracy of centernet with resnet backbone is very poor.