return the elements in sorted order
Keyword Arguments:
      out (tuple, optional) -- the output tuple of (Tensor,
      LongTensor) that can be optionally given to be used as output
      buffers
Example:
  >>> x = torch.arange(1., 6.)
  >>> x
  tensor([ 1.,  2.,  3.,  4.,  5.])
  >>> torch.topk(x, 3)
  torch.return_types.topk(values=tensor([5., 4., 3.]), indices=tensor([4, 3, 2]))
