Fix flash_attention.py: wrong argument passing for attn_implementation (#41347)
* Fix flash_attention.py: wrong argument passing for attn_implementation
The name of the attn type argument for `_flash_attention_forward()` should be `implementation`, instead of `attn_implementation` which currently uses in the function call. This would result in wrong type specification.
* modify the kwargs inside _flash_attention_forward
* fix the doc
* fix typo
---------
Co-authored-by: Cyril Vallez <cyril.vallez@gmail.com>