[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

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    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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• Testing Strategy
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