aligned sample_beam output selection with beam_search
Author: hukuda222
Pull request: yes

# What does this PR do?

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Fixes #25363

The current `sample_beam` executes the same process `num_return_sequences` times when `num_return_sequences>1`. So, the exact same output may be produced in a single run.

To solve the above problem, I modified the output of `sample_beam` to select from beam candidates as in the normal `beam_search`.


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@gante 

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

cc @gante

~~It appears that some test needs to be changed. Please review when that is done.~~ done

@sgugger context: beam sample with `num_return_sequences>1`, as it was coded, was equivalent to independently calling beam sample `num_return_sequences` times, returning the best beam in each call.

This was inconsistent with other beam methods, where `num_return_sequences>1` returns as many beams. This PR fixes it. 

(and perhaps not as important: TF and FLAX already had the "correct" version of beam sample when `num_return_sequences>1`)