"'none'": no reduction will be applied, "'mean'": the sum of
        the output will be divided by the number of elements in the
        output, "'sum'": the output will be summed. Note:
        "size_average" and "reduce" are in the process of being
        deprecated, and in the meantime, specifying either of those
        two args will override "reduction". Default: "'mean'"
Return type:
      Tensor
Example:
  >>> # input is of size N x C = 3 x 5
  >>> input = torch.randn(3, 5, requires_grad=True)
  >>> # each element in target has to have 0 <= value < C
  >>> target = torch.tensor([1, 0, 4])
  >>> output = F.nll_loss(F.log_softmax(input, dim=1), target)
  >>> output.backward()
