Plot functions for tensorboard data

[michaeldeistler]

Tensorboard can be a bit of a hurdle to debug NN training. Instead, we might want to have functions such as:

from sbi.analysis import plot_loss

_ = inference.append_simulations(theta, x).train()
plot_loss(inference)

This function reads the logged loss from the SummaryWriter and plots it.

[lappalainenj]

According to this function, _summarize, we log the following:

            - epochs_trained
            - best_validation_log_prob
            - validation_log_probs_across_rounds
            - train_log_probs_across_rounds
            - epoch_durations_sec_across_rounds

What would be important to plot? What do we mean with across rounds? Because the metric is apparently logged over epochs.