Fix `test_orpo_trainer_demo.py`

[alvarobartt]

Description

This PR fixes the test_orpo_trainer_demo.py as it was overriding the value of sample["chosen"] leading to a bad formatting within the prompt.

Additionally, this PR also applies the following minor changes related to formatting and readability:

• Place everything under if __name__ == "__main__"
• Remove unused os import and duplicated transformers imports
• Add reference to original TRL script

[jiwooya1000]

Thank you for the fix and clarification @alvarobartt😃