Add a callback to log raw stats

[vwxyzjn]

Describe

SB3 only logs the mean episodic return (i.e., rollout/ep_rew_mean), which is good enough in most cases. However, sometimes we may want the raw episodic return that is unaveraged. In my case, I was trying to compare my raw episodic return with rollout/ep_rew_mean, and it would look like something below, where SB3's orange curve would be much smoother because of the average operation.

Maybe it's worth also logging the raw stats, or at least give an option to.

#216 tracks the progress.

[araffin]

closed in #216