Scale the coords in `flow` to -1/1 or the image size depending on `to_unit`.

def scale_flow(flow, to_unit=True):
    "Scale the coords in `flow` to -1/1 or the image size depending on `to_unit`."
    s = tensor([flow.size[0]/2,flow.size[1]/2])[None]
    if to_unit: flow.flow = flow.flow/s-1
    else:       flow.flow = (flow.flow+1)*s
    return flow