Applies the inverse affine transform described in `m` to `c`.

def _affine_inv_mult(c, m):
    "Applies the inverse affine transform described in `m` to `c`."
    size = c.flow.size()
    h,w = c.size
    m[0,1] *= h/w
    m[1,0] *= w/h
    c.flow = c.flow.view(-1,2)
    a = torch.inverse(m[:2,:2].t())
    c.flow = torch.mm(c.flow - m[:2,2], a).view(size)
    return c