Change Dataset's repr to use angled brackets

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Why are these changes needed?

This PR updates Dataset's __repr()__ to use angled brackets. Since the repr of a Dataset doesn't contain the values, eval(repr(...)) of a Dataset clearly doesn't yield an object with the same value. Therefore, it should be using angled brackets according to the official python docs https://docs.python.org/3/library/functions.html#repr

Related issue number

This addresses #28178, and is related to #31604

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 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 :(

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Changed unit tests are passing locally, and it looks correct in a small example.

[feefs]

I wanted to get a review from only @bveeramani, but I couldn't change the reviewers. Sorry for the spam :)

Not sure if the regex find and replace I did covers the third checkbox for "including any doc changes". I'll wait for the CI test results before touching the fourth checkbox.

@bveeramani I also noticed while editing that DatasetPipeline is in a similar situation, should it be tackled as well?