symbolic_trace: add past_key_values, llama, sdpa support (#28447)

* torch.fx: add pkv, llama, sdpa support

* Update src/transformers/models/opt/modeling_opt.py

* remove spaces

* trigger ci

* use explicit variable names

tests/models/llama/test_modeling_llama.py

@@ -292,6 +292,7 @@ class LlamaModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixi
     )
     test_headmasking = False
     test_pruning = False
+    fx_compatible = True
 
     def setUp(self):
         self.model_tester = LlamaModelTester(self)