[Data] Fix duplicate execution in dataset.union

[liuxsh9]

Why are these changes needed?

When using dataset.union, we observed a duplicate execution, as demonstrated by the reproducing script and its execution timeline.

import ray
import time

def fun(batch):
    time.sleep(1)
    return batch

ray.init()
ds0 = ray.data.from_items([i for i in range(10)]).repartition(10)
ds0 = ds0.map_batches(fun, concurrency=1)

ds1 = ray.data.from_items([i for i in range(10)]).repartition(10)
ds1 = ds1.map_batches(fun, concurrency=1)

ds = ds0.union(ds1)
ds = ds.map_batches(fun, concurrency=1)
ds = ds.materialize()

This PR aims to fix this issue. After:

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 :(

[liuxsh9]

@anyscalesam Thanks to #45860 , the issue has been resolved. We can now close this.