[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

tests/models/gpt2/test_modeling_gpt2.py

@@ -832,7 +832,8 @@ class GPT2ModelLanguageGenerationTest(unittest.TestCase):
         start = datetime.datetime.now()
         model.generate(input_ids, do_sample=False, max_time=None, max_length=256)
         duration = datetime.datetime.now() - start
-        self.assertGreater(duration, datetime.timedelta(seconds=1.5 * MAX_TIME))
+        self.assertGreater(duration, datetime.timedelta(seconds=MAX_TIME))
+        self.assertLess(duration, datetime.timedelta(seconds=1.5 * MAX_TIME))
 
     @slow
     def test_contrastive_search_gpt2(self):