Fix: prevent .gitignore truncation in run_clm_no_trainer.py

[luaenrique]

What does this PR do?

This PR fixes an issue in run_clm_no_trainer.py where the .gitignore file was being truncated due to the use of "w+" mode when writing.
Now, the script preserves existing .gitignore content, avoids duplicates, and safely appends the required ignore patterns for model outputs (step_* and epoch_*).

I have tested with:

1. no gitignore: file was created and populated with step_* and epoch_*
2. gitignore with additional content: step_* and epoch_* were added after previous content
3. blank gitignore: step_* and epoch_* were added to the file

Fixes #41896

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wdyt @Rocketknight1?

[Rocketknight1]

Yes, nice fix! This looks like it actually does what the original code was supposed to do. Thank you!

[HuggingFaceDocBuilderDev]

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[luaenrique]

Thanks @Rocketknight1!