[RLlib - Offline RL] Allow incomplete SampleBatch data and fully compressed observations.

[simonsays1980]

Why are these changes needed?

Some users have observations in their old SampleBatch data that are fully compressed, i.e. compression is over all timesteps (instead of compressing each single observation in a list). These observations cannot be read in the actual state of the OfflinePreLearner. Furthermore, if experiences are incomplete, SingleAgentEpisodes cannot be built.

This PR suggests multiple small changes to the _map_sample_batches_to_episodes to enable reading old SampleBatch data with incomplete experiences and/or fully compressed observations.

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[sven1977]

Very nice PR! Thanks @simonsays1980!

A handful of nits and one question. Otherwise, good to go.