Randomly flip `x` image based on `k`.

def _dihedral(x, k:partial(uniform_int,0,7)):
    "Randomly flip `x` image based on `k`."
    flips=[]
    if k&1: flips.append(1)
    if k&2: flips.append(2)
    if flips: x = torch.flip(x,flips)
    if k&4: x = x.transpose(1,2)
    return x.contiguous()