(int) the index at which to insert the singleton dimension
unsqueeze(input, dim) -> Tensor
Returns a new tensor with a dimension of size one inserted at the
specified position.
The returned tensor shares the same underlying data with this tensor.
A dim value within the range [-input.dim() - 1, input.dim() + 1)
can be used. Negative dim will correspond to unsqueeze
applied at dim = dim + input.dim() + 1.