TPUSpawn + IterableDataset error message

[ethanwharris]

What does this PR do?

Adds a more firendly error message when using iterable dataset + TPUSpawn (currently this fails inside torch XLA here: https://github.com/pytorch/xla/blob/86463f894eb21f31dcd3449e4e55cc45b8a8586a/torch_xla/distributed/parallel_loader.py#L89)

See example outputs here: https://colab.research.google.com/drive/1fXtIfIemyzuF2Flxvyjd0aHz6zL8u5AE?usp=sharing

Would be nice if this could happen on just one rank (perhaps switching to rank_zero_warn?), can't happen pre-accelerator as needs the dataloaders to actually exist

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[ethanwharris]

@tchaton Thanks for the review 😃 I've added an earlier message when the dataloaders are passed to fit and some tests, mind re-reviewing?

[kaushikb11]

LGTM!