[WIP] Dataset iterator

[stephanie-wang]

Why are these changes needed?

TODO: Docs and deprecation warnings for APIs.

Related issue number

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• [ ] I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• [ ] I've run scripts/format.sh to lint the changes in this PR.
• [ ] I've included any doc changes needed for https://docs.ray.io/en/master/.
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• Testing Strategy
  • [ ] Unit tests
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[matthewdeng]

@amogkam @clarkzinzow @bveeramani assigning to you all to review the API!

[amogkam]

Are we still planning to remove iter_batches from the base Dataset API and instead have ds.to_iterator().iter_batches()?

Basically avoiding duplicate APIs in both Dataset and DatasetIterator

[stephanie-wang]

Are we still planning to remove iter_batches from the base Dataset API and instead have ds.to_iterator().iter_batches()?

Basically avoiding duplicate APIs in both Dataset and DatasetIterator

Eventually yes, but my opinion on this is that we should wait for at least a full release cycle so we can first stabilize the DatasetIterator API.