revert behavior of _prepare_from_posids
Author: winglian
Pull request: yes
Labels: name: Flash Attention

# What does this PR do?

#39474 causes the grad norm to become NaN during training. We don't need to revert the whole thing, but reverting just this function seems to fix the regression.

@ArthurZucker @SunMarc 

pre-#39474 loss
<img width="957" height="496" alt="Screenshot 2025-07-23 at 5 28 24 PM" src="https://github.com/user-attachments/assets/f319791b-4f3b-489a-b7dd-dc0e2eb3112f" />

post-#39474 loss
<img width="1290" height="282" alt="Screenshot 2025-07-23 at 5 31 19 PM" src="https://github.com/user-attachments/assets/ee5fe4f9-5c42-4c7e-8f78-c6e14f960737" />

with this fix
<img width="1000" height="301" alt="Screenshot 2025-07-23 at 7 07 01 PM" src="https://github.com/user-attachments/assets/1dec04d5-5378-4f05-b57d-8a81a403ec28" />

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Comments:
(Thanks a lot for checking sorry for breaking)