Create a Dataset from Tensors

[vlomonaco]

At the moment we support the creation of benchmarks from tensors. However, it would be nice to support also the creation of a dataset from tensors. This is already done in a very shallow way by Pytorch with TensorDataset.

@lrzpellegrini this won't be supported by the current AvalancheDataset, am I right?

[lrzpellegrini]

We have AvalancheTensorDataset ;)

[vlomonaco]

ahah we definitely need to update the doc... these are useful features difficult to notice from the outside.