Iterable Datasets work with TPUs

[gkroiz]

Bug description

The error TPUs do not currently support IterableDataset objects, the dataset must implement len. HINT: You can mock the length on your dataset to bypass this error. is raised when using IterableDatasets on TPUs (code location for fabric and pytorch). At the time this error was introduced (#6875), there was a line in the xla code that would call for len(). However, this issue from the xla side was fixed shortly after in pytorch/xla#2866. #6955 was then submitted to request a fix for the error check, but was never resolved. Since #6955 is from two years ago and I am unable to open the issue, I wanted to revive this topic in a new issue.

For reference, I ran into this warning when running the dqn lightning tutorial on TPU(s).

Unless there is a good reason to keep this error check, can we either remove it entirely or adjust it to a warning saying something along the lines of "If you want to use len() on your IterableDataset, you will need to manually implement len()?

What version are you seeing the problem on?

1.6.x, 1.7.x, 1.8.x, 1.9.x, 2.0+

cc @Borda @JackCaoG @steventk-g @Liyang90 @carmocca

[carmocca]

Hi! I think we can just remove this check. Would you like to open a PR?