text: Title: Shape of pred & label is difference when training siamrpn_alex_dwxcorr/config.yaml 
  Body: Hello every one,  I had trained siamrpn_r50_l234_dwxcorr/config.yaml with my PC successfully.
  But when I train with siamrpn_alex_dwxcorr/config.yaml, it always raises Error during computing loss function. 
  
  After tracing, in the following function, I found that shape of pred & label is difference, pred is  (17x17) and label is (25x25). which is not the case for Alex Backbone. Should I change any other configuration before training with "--cfg=(path)/siamrpn_alex_dwxcorr/config.yaml" ? Thank you
  
  def select_cross_entropy_loss(pred, label):
      pred = pred.view(-1, 2)
      label = label.view(-1)
      pos = label.data.eq(1).nonzero().squeeze().cuda()
      neg = label.data.eq(0).nonzero().squeeze().cuda()
      loss_pos = get_cls_loss(pred, label, pos)
      loss_neg = get_cls_loss(pred, label, neg)
      return loss_pos * 0.5 + loss_neg * 0.5
  