Fix usage of unpad_input function (#35925)
Fix usage of unpad_function
See https://github.com/huggingface/transformers/issues/35899
In the [commit](https://github.com/Dao-AILab/flash-attention/commit/cdbbe844b1c0bcba3362e1f8c8af4d6f6d0bf300) return type of `unpad_input` was changed.
Now the code support older and newer versions
Co-authored-by: Pavel Gein <pavel.gein@gmail.com>
