w.r.t. "input" will be a sparse tensor.
  * **out** (*Tensor**, **optional*) -- the destination tensor

Example:
  >>> t = torch.tensor([[1, 2], [3, 4]])
  >>> torch.gather(t, 1, torch.tensor([[0, 0], [1, 0]]))
  tensor([[ 1,  1],
          [ 4,  3]])
