Add UnconditionalDistribution and UnconditionalTransform

This PR replaces the `Unconditional` class with two dedicated classes: `UnconditionalDistribution` and `UnconditionalTransform`. This should resolve some issues with type checkers (see https://github.com/sbi-dev/sbi/pull/1099). The `Unconditional` class is deprecated and will be removed in the future. Using it raises a warning.