If "as_tuple" is "True", one 1-D tensor for each dimension,
      containing the indices of each nonzero element along that
      dimension.
Return type:
      LongTensor or tuple of LongTensor
Example:
  >>> torch.nonzero(torch.tensor([1, 1, 1, 0, 1]))
  tensor([[ 0],
          [ 1],
          [ 2],
          [ 4]])
  >>> torch.nonzero(torch.tensor([[0.6, 0.0, 0.0, 0.0],
  ...                             [0.0, 0.4, 0.0, 0.0],
  ...                             [0.0, 0.0, 1.2, 0.0],
  ...                             [0.0, 0.0, 0.0,-0.4]]))
  tensor([[ 0,  0],
          [ 1,  1],
          [ 2,  2],
          [ 3,  3]])
  >>> torch.nonzero(torch.tensor([1, 1, 1, 0, 1]), as_tuple=True)
  (tensor([0, 1, 2, 4]),)
  >>> torch.nonzero(torch.tensor([[0.6, 0.0, 0.0, 0.0],
  ...                             [0.0, 0.4, 0.0, 0.0],
  ...                             [0.0, 0.0, 1.2, 0.0],
