append to "input" along "dim" before computing the difference.
        Their dimensions must be equivalent to that of input, and
        their shapes must match input's shape except on "dim".
Keyword Arguments:
      out (Tensor, optional) -- the output tensor.
Example:
  >>> a = torch.tensor([1, 3, 2])
  >>> torch.diff(a)
  tensor([ 2, -1])
  >>> b = torch.tensor([4, 5])
  >>> torch.diff(a, append=b)
  tensor([ 2, -1,  2,  1])
  >>> c = torch.tensor([[1, 2, 3], [3, 4, 5]])
  >>> torch.diff(c, dim=0)
  tensor([[2, 2, 2]])
  >>> torch.diff(c, dim=1)
  tensor([[1, 1],
          [1, 1]])
