Pad `x` with `padding` pixels. `mode` fills in space ('zeros','reflection','border').

def _pad_default(x, padding:int, mode='reflection'):
    "Pad `x` with `padding` pixels. `mode` fills in space ('zeros','reflection','border')."
    mode = _pad_mode_convert[mode]
    return F.pad(x[None], (padding,)*4, mode=mode)[0]