[DataLoader] Add context to NotImplementedErrors in dataset.py (#100667)
Add helpful context message to `NotImplementedError`'s thrown by Dataset and IterableDataset, reminding users that they must implement `__getitem__`/`__iter__` in subclasses. Currently, users are presented with a bare `NotImplementedError` without describing the remedy.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100667
Approved by: https://github.com/NivekT
