Function calculate_padding_out 

pub fn calculate_padding_out(
    kernel_size: usize,
    stride: usize,
    padding: usize,
    dilation: usize,
    size_in: usize,
    size_out: usize,
) -> usize

Compute the padding_out for a transpose conv that exactly recovers the original size_in from size_out, accounting for any input elements the forward conv dropped. Shared by conv{1,2,3}d_x_backward and the CubeCL dgrad fallback so the two paths can’t drift.