Refactor and improve BetterTransformerIntegrationTest @tejhande
Author: tejhande
Pull request: yes

Refactor unit tests for `to_bettertransformer()` and `reverse_bettertransformer()` methods in `BetterTransformerIntegrationTest`.

- Cleaned up imports and added clarifying comments.
- Improved temporary directory handling for model saving and reloading.
- Ensured compatibility with latest `transformers` and `torch` dependencies.
- Verified robust testing of model state transitions.

# What does this PR do?

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Fixes # (issue)


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Comments:
@fxmarty I believe you were involved in the addition of BetterTransformers?