retained or not.
Keyword Arguments:
      out (Tensor, optional) -- the output tensor.
Example:
  >>> a = torch.randn(3, 3)
  >>> torch.logsumexp(a, 1)
  tensor([1.4907, 1.0593, 1.5696])
  >>> torch.dist(torch.logsumexp(a, 1), torch.log(torch.sum(torch.exp(a), 1)))
  tensor(1.6859e-07)
