Add `dim` parameter to `LabelTensor.stack` method

[GiovanniCanali]

Is your feature request related to a problem? Please describe.
The current implementation of the stack method in the LabelTensor class does not allow users to specify the dimension along which tensors are stacked. Since the method internally relies on torch.stack, it would be beneficial to expose this parameter to provide greater flexibility and consistency with the underlying PyTorch API.

Note: An additional check may be necessary. To preserve label integrity, the newly introduced dimension should likely not be the last one.