[Examples][Flax] added test file in summarization example

[bhadreshpsavani]

What does this PR do?

Fixes #12527

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@sgugger, @patil-suraj

[patil-suraj]

Thanks for fixing this!