[Train] Remove references to DatasetPipeline

[bveeramani]

Why are these changes needed?

DatasetPipeline has been soft deprecated since Ray 2.6, and is being hard deprecated in Ray 2.8. Accordingly, this PR removes references to it in the Train code.

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/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/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 at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
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  • This PR is not tested :(

[justinvyu]

Very nice 🤩

[matthewdeng]

nice