Storage policy's buffer doesn't keep the targets attribute

All storage policies in Avalanche initialize their internal buffer with an empty dataset, i.e.:
```python
self._buffer: AvalancheDataset = concat_datasets([])
```
When a new dataset is used to update the current buffer, `self._buffer` is concatenated with the new dataset, and since it's a concatenation of an empty dataset with another dataset , the targets are not transferred in the case of classification datasets.

Is this a bug or is it the expected behavior?