Fixes default value of `softmax_scale` in `PhiFlashAttention2`.
Author: gugarosa
Pull request: yes

# What does this PR do?

- Phi has never used `softmax_scale=1.0` with Flash-Attention, so the default is being moved to `None`. This tentatively fixes any issue regarding fine-tuning Phi-based checkpoints when Flash-Attention 2 is turned on.

- Documentation is also updated to reflect the official Phi checkpoints.

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Fixes #28488 (tentative)


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Comments:
The `loss=0.0` error while fine-tuning with FP16 is another issue and I do have an ugly fix, but will look into it with more patience (and use a separate PR).

No problems! Thanks for the merge!

Thanks very much @gugarosa for the deep dive and the fix! 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_28537). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.