Fix KeyError for flash_attn in import_utils.py on Python 3.13Fix

[aryanp2107]

What does this PR do?

Fixes #45520

When flash_attn is not installed, direct dictionary access on
PACKAGE_DISTRIBUTION_MAPPING["flash_attn"] throws a KeyError
because the key doesn't exist in the dictionary returned by
importlib.metadata.packages_distributions(). This affects Python 3.13
environments where Flash Attention is not available.

Replaced direct dictionary access with .get() and an empty list default
on four lines (951, 970, 982, 993), so the list comprehension iterates
over an empty list instead of crashing. This is consistent with the safe
access pattern already used in _is_package_available() at line 59.

• [ x ] I confirm that this is not a pure code agent PR.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ x ] Did you read the contributor guideline,
  Pull Request section?
• [ x ] Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• 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?

@ivarflakstad (listed for AMD ROCm, relevant since the issue was reported on ROCm)

[github-actions]

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=45650&sha=c0b6ec

[Rocketknight1]

There was already an existing PR, please check first before firing your code agent at issues! #45524