ExperimentDataPipe should record the iterated joinids

[ebezzi]

When ExperimentDataPipe iterates through cells, the obs_joinids are not recorded anywhere. This isn't important for training but it's necessary if the same datapipe is used for a forward pass (e.g. when generating embeddings). The current _obs_joinids field can be used but:

1. It requires shuffling to be off.
2. Doesn't work with multiple workers, since they don't process in order.