shape as output or output.size(dim), if dim was specified)
           representing the number of occurrences for each unique
           value or tensor.
Return type:
      (Tensor, Tensor (optional), Tensor (optional))
Example:
  >>> output = torch.unique(torch.tensor([1, 3, 2, 3], dtype=torch.long))
  >>> output
  tensor([1, 2, 3])

  >>> output, inverse_indices = torch.unique(
  ...     torch.tensor([1, 3, 2, 3], dtype=torch.long), sorted=True, return_inverse=True)
  >>> output
  tensor([1, 2, 3])
  >>> inverse_indices
  tensor([0, 2, 1, 2])

  >>> output, inverse_indices = torch.unique(
  ...     torch.tensor([[1, 3], [2, 3]], dtype=torch.long), sorted=True, return_inverse=True)
  >>> output
  tensor([1, 2, 3])
  >>> inverse_indices
  tensor([[0, 2],
          [1, 2]])
