code: def z2polar(edges):
      z = edges.dst["xy"] - edges.src["xy"]
      rho = th.norm(z, dim=-1, p=2)
      x, y = z.unbind(dim=-1)
      phi = th.atan2(y, x)
      return {"u": th.cat([rho.unsqueeze(-1), phi.unsqueeze(-1)], -1)}
  
label_desc: Use image node_id 2 for calling a global function with example usage: z2polar(edges) and returns: dict