[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

docs/source/en/perf_infer_gpu_one.md

@@ -201,6 +201,7 @@ For now, Transformers supports SDPA inference and training for the following arc
 * [Dpr](https://huggingface.co/docs/transformers/model_doc/dpr#transformers.DprReader)
 * [Falcon](https://huggingface.co/docs/transformers/model_doc/falcon#transformers.FalconModel)
 * [Gemma](https://huggingface.co/docs/transformers/model_doc/gemma#transformers.GemmaModel)
+* [GPT2](https://huggingface.co/docs/transformers/model_doc/gpt2)
 * [GPTBigCode](https://huggingface.co/docs/transformers/model_doc/gpt_bigcode#transformers.GPTBigCodeModel)
 * [JetMoe](https://huggingface.co/docs/transformers/model_doc/jetmoe#transformers.JetMoeModel)
 * [Jamba](https://huggingface.co/docs/transformers/model_doc/jamba#transformers.JambaModel)