Use `torch.nested_tensor` for arrays of varying length in torch formatter
Use `torch.nested_tensor` for arrays of varying length in `TorchFormatter`.

The PyTorch API of nested tensors is in the prototype stage, so wait for it to become more mature.
What's the current behavior? Currently, we return a list of Torch tensors if their shapes don't match. If they do, we consolidate them into a single Torch tensor.