[RLlib; Offline RL] - Validate episodes before adding them to the buffer.

[simonsays1980]

Why are these changes needed?

At the moment the OfflinePreLearner samples recorded episodes or SampleBatches from a ray.data dataset and then adds them to a buffer which corrdinates the time step sampling. It could potentially happen that

1. The sampled episodes are duplicates (either in the batch or in the buffer)
2. The sampled episodes are not terminated nor truncated and therefore could and certainly will be fragmented in time order (i.e. we maybe sample first an episode chunk that contains timesteps 11 to 21 before we sample 0 to 11).

In both cases the buffer would raise an error as soon as SingleAgentEpisode.concat is called.

This PR introduces a _validate_episodes method to the OfflinePreLearner to check episodes for duplicates and fragments and returns only unique episodes that are not in the buffer, yet. It disallows uncompleted episodes and thereby ensures that no fragments are added. Users are responsible to record only full episodes.

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