[DataLoader2] Skip wrapping if serialization wrapper attached & deepcopy DataPipe in load_state_dict

[ejguan]

Per title

• We don't need to attach serialization wrapper if the last DataPipe has been _DataPipeSerializationWrapper
• When we load_state_dict, we still need a copy of self.datapipe as it has been done in __init__ function. We should either do deepcoy to self._datapipe_before_reading_service_adapt or skip deepcopy to self._datapipe_before_reading_service_adapt at those two places.

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[ejguan]

LGTM! Question: do we expect any case where the datapipe being passed in already has a wrapper?

It's more like a future proof. (Technically, if users manually deserialize the DataPipe state, the returned DataPipe is wrapped by the wrapper.