[`GPT2`] Add SDPA support (#31172)
* `gpt2` sdpa support
* fix (at least) one test, style, repo consistency
* fix sdpa mask in forward --> fixes generation
* test
* test2
* test3
* test4
* simplify shapes for attn mask creation and small comments
* hub fail test
* benchmarks
* flash attn 2 mask should not be inverted on enc-dec setup
* fix comment
* apply some suggestion from code review
- only save _attn_implentation once
- remove unnecessary comment
* change elif logic
* [run-slow] gpt2
* modify `test_gpt2_sample_max_time` to follow previous assertion patterns
