[Data] Hard deprecate DatasetPipeline

[bveeramani]

Stacked on:

• [Train/Data] Deprecate legacy DatasetConfig #39963
• [Data] Remove DatasetPipeline unit tests #40133
• [Data] Remove DatasetPipeline from nightly tests #40130
• [RLlib] Don't call deprecated Dataset.repeat in DatasetReader #40161

Why are these changes needed?

Fixes #40166.

Related issue number

Checks

• 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
  • Release tests
  • This PR is not tested :(

[c21]

Thanks @bveeramani, great job!

[c21]

LGTM

[scottjlee]

Thanks! so we will not be removing the DatasetPipeline class completely for now, just raising the deprecation warning?

[c21]

so we will not be removing the DatasetPipeline class completely for now, just raising the deprecation warning?

The only thing I am worried about is the usage of DatasetPipeline.from_iterable. I want to avoid the situation that Python throws the module/class not found, and people shocked about it. Any blocker if we keep the DatasetPipeline class around? All actual code inside DatasetPipeline is removed.