Grows the spatial dims by (dy, dx) cells of value on the high
side only: the shape-alignment pad (e.g. to a /32-divisible U-Net
input) that a later slice removes exactly.
Usage
g_pad_rb(x, dy, dx, value =0)
Arguments
x
Array of rank >= 2 (traced or plain).
dy, dx
Non-negative cell counts to add below / to the right.
