Update conditional probability function

[Sohl-Dickstein]

• Rename conditional_prob_fn to cond_log_prob_fn.
• Add keyword to determine whether probabilities are normalized to sum to 1 over targets, or whether probabilities are absolute. Normalizing to 1 over targets allows a broader class of models to return conditional log probabilities, including energy based models, and human evaluation.
• Edit a bunch of doc strings.