CC@neilshah12 and@richardliaw

Why are these changes needed?

This allows Ray Data users to callds.offset on their datasets. This feature corresponds to SQL'sOFFSET feature.This feature is useful, e.g., for implementing pagination on a dataset (in conjunction withds.limit).

Notes

• should we detectoffset < 0 and raiseValueError?
• should we detectoffset is None and raiseValueError?
  • another option: detectNone and treat this asoffset == 0. This more closely aligns with SQL syntax.
• are we missing any test coverage?

Related issue number

n/a

Checks

• I've signed off every commit(by using the -s flag, i.e.,git commit -s) in this PR.
• I've runscripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed forhttps://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it indoc/source/tune/api/ under the
    corresponding.rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures athttps://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(