[`FA2`] Add flash attention for opt (#26414)
* added flash attention for opt
* added to list
* fix use cache (#3)
* style fix
* fix text
* test fix2
* reverted until 689f599
* torch fx tests are working now!
* small fix
* added TODO docstring
* changes
* comments and .md file modification
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Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>
