[Feature Request] Default creation of LazyStackedTensorDict vs TensorDict when calling torch.stack?

[vmoens]

By default, torch.stack(list_of_tds, dim) will return a LazyStackedTensorDict object.
Keeping the discussion about the usefulness of this class apart, it may be more desirable to have a LazyStackedTensorDict created only when it is explicitely asked by the user, e.g.

stacked_td = torch.stack(list_of_tds, dim)
assert isinstance(stacked_td, TensorDict) # passes

stacked_td = torch.stack(list_of_tds, dim, contiguous=False)
assert isinstance(stacked_td, LazyStackedTensorDict) # passes

The main advantage is that working with LazyStackedTensorDict can be computationally expensive and users may not be aware of this bottleneck.
Also, when called on lists of torch.Tensor objects, torch.stack does not return a non-contiguous object. Aligning TensorDict behaviour to this might be beneficial for consistency purposes.

[vmoens]

Closing as #352 made it clear that this is needed. Happy to re open if needed