compute_vertex_normals is buggy

[JunzheJosephZhu]

    d0 = d0 / torch.norm(d0)
    d1 = v[(i + 2) % 3] - v[i]
    d1 = d1 / torch.norm(d1)
    d = torch.sum(d0*d1, 0)

Here, the norm is not calculated per-vertex