Parameters:
      * dim (int) -- dimension along which to index
  * **index** (*Tensor*) -- indices of "source" to select from,
    should have dtype either *torch.int64* or *torch.int32*

  * **source** (*Tensor*) -- the tensor containing values to add

Keyword Arguments:
      alpha (Number) -- the scalar multiplier for "source"
Example:
  >>> x = torch.ones(5, 3)
  >>> t = torch.tensor([[1, 2, 3], [4, 5, 6], [7, 8, 9]], dtype=torch.float)
  >>> index = torch.tensor([0, 4, 2])
  >>> x.index_add_(0, index, t)
  tensor([[  2.,   3.,   4.],
          [  1.,   1.,   1.],
          [  8.,   9.,  10.],
          [  1.,   1.,   1.],
          [  5.,   6.,   7.]])
  >>> x.index_add_(0, index, t, alpha=-1)
  tensor([[  1.,   1.,   1.],
          [  1.,   1.,   1.],
          [  1.,   1.,   1.],
          [  1.,   1.,   1.],
          [  1.,   1.,   1.]])
