the last index changing the fastest (C-style).
If "input" has n dimensions, then the resulting indices tensor
   "out" is of size (z \times n), where z is the total number of non-
   zero elements in the "input" tensor.
When "as_tuple" is "True":
Returns a tuple of 1-D tensors, one for each dimension in "input",
   each containing the indices (in that dimension) of all non-zero
   elements of "input" .
If "input" has n dimensions, then the resulting tuple contains n
   tensors of size z, where z is the total number of non-zero elements
   in the "input" tensor.
As a special case, when "input" has zero dimensions and a nonzero
   scalar value, it is treated as a one-dimensional tensor with one
   element.
Parameters:
      input (Tensor) -- the input tensor.
Keyword Arguments:
      out (LongTensor, optional) -- the output tensor
      containing indices
Returns:
      If "as_tuple" is "False", the output tensor containing indices.