def merge_bn(model):
    ''' Kai Zhang, 11/Jan/2019.
    merge all 'Conv+BN' (or 'TConv+BN') into 'Conv' (or 'TConv')
    based on https://github.com/pytorch/pytorch/pull/901
    '''
    prev_m = None
    for k, m in list(model.named_children()):
        if (isinstance(m, nn.BatchNorm2d) or isinstance(m, nn.BatchNorm1d)) and (isinstance(prev_m, nn.Conv2d) or isinstance(prev_m, nn.Linear) or isinstance(prev_m, nn.ConvTranspose2d)):

            w = prev_m.weight.data

            if prev_m.bias is None:
                zeros = torch.Tensor(prev_m.out_channels).zero_().type(w.type())
                prev_m.bias = nn.Parameter(zeros)
            b = prev_m.bias.data

            invstd = m.running_var.clone().add_(m.eps).pow_(-0.5)
            if isinstance(prev_m, nn.ConvTranspose2d):
                w.mul_(invstd.view(1, w.size(1), 1, 1).expand_as(w))
            else:
                w.mul_(invstd.view(w.size(0), 1, 1, 1).expand_as(w))
            b.add_(-m.running_mean).mul_(invstd)
            if m.affine:
                if isinstance(prev_m, nn.ConvTranspose2d):
                    w.mul_(m.weight.data.view(1, w.size(1), 1, 1).expand_as(w))
                else:
                    w.mul_(m.weight.data.view(w.size(0), 1, 1, 1).expand_as(w))
                b.mul_(m.weight.data).add_(m.bias.data)

            del model._modules[k]
        prev_m = m
        merge_bn(m)
 Kai Zhang, 11/Jan/2019.
    merge all 'Conv+BN' (or 'TConv+BN') into 'Conv' (or 'TConv')
    based on https://github.com/pytorch/pytorch/pull/901
    
utils/utils_bnorm.py