The body of de_log_lik_iid()'s default method, with the per-pair scorer
left open so nre() can reuse it. score(de, x_rows, theta_rows) is
de_log_prob() for a density estimator and
nre_score() for a ratio estimator.
The body of de_log_lik_iid()'s default method, with the per-pair scorer
left open so nre() can reuse it. score(de, x_rows, theta_rows) is
de_log_prob() for a density estimator and
nre_score() for a ratio estimator.