Add sample_dim for condition in conditional estimator

[janfb]

In our current implementation, we do not allow the condition to have a sample_dim (only a batch_dim). It would be nice to have a sample_dim as well, e.g., for post-hoc conditioning on a batch of condition values that match a set of i.i.d. x (sample_dim).

See e.g., #1331, where a batch of conditions has to be the same for all entries in a batch of non-iid xs, i.e., it's not possible to have both a batch of iid samples of a batch of xs, that each have their own corresponding batch of iid conditions.