Compute and log variances of the log probs

[Alex]

One more thing: I'd add computing and tracking two variances of the log probs:

1. variance over samples of logprobs_estimates (to understand better the behaviour of the correlation coefficient over the training)
2. median over samples of the variances of the logprobs_estimates over trajectories for each sample (to get a sense of how noisy the estimation is). The math is a bit tricky here as we use log mean as an estimation, not just the mean. But there're some work around: https://stats.stackexchange.com/questions/418313/variance-of-x-and-variance-of-logx-how-to-relate-them
   But in any case, we will need to compute empirical var(P_F(tau) / P_B (tau)) / n_traj for each sample and then play around a bit with it to get variance for the log mean estimation.

Originally posted by @AlexandraVolokhova in #167 (comment)