XLA + IterableDataset support should now work on XLA master

[ethanwharris]

🐛 Bug

Following pytorch/xla#2866 XLA should now support IterableDatasets (or at least not fail on trying to get the length). This means we can edit the check from #6875 to only fail with older XLA versions.

Note: the fix from pytorch/xla#2866 has not yet been released

Please reproduce using the BoringModel

This message from PL slack, currently gives an error: https://pytorch-lightning.slack.com/archives/CQXV8BRH9/p1618028942117600?thread_ts=1617801747.099100&cid=CQXV8BRH9

[stale]

This issue has been automatically marked as stale because it hasn't had any recent activity. This issue will be closed in 7 days if no further activity occurs. Thank you for your contributions, Pytorch Lightning Team!

[stale]

This issue has been automatically marked as stale because it hasn't had any recent activity. This issue will be closed in 7 days if no further activity occurs. Thank you for your contributions, Pytorch Lightning Team!