'Clip boxes of the pad area.
:param tensor: [n, c, H, W]
:param pad_shape: [h, w]
:return: [n, c, h, w]'

@staticmethod
def _clip_pad(tensor, pad_shape):

    (H, W) = tensor.shape[2:]
    (h, w) = pad_shape
    if ((h < H) or (w < W)):
        tensor = tensor[:, :, :h, :w].copy()
    return tensor
