// perform backward data 1x3 convolution (grad_out * w_rot180) on grad_out2 input of shape [N,1,W,C] with padding=(0,1) to produce output of shape [N,1,W,C]
// perform backward data 3x3 convolution (grad_out * w_rot180) on grad_out2 input of shape [N,3,W,C] with padding=(1,1) to produce output of shape [N,3,W,C]