Gemma2: eager attention by default (#32865)

src/transformers/models/gemma2/modeling_gemma2.py

@@ -656,6 +656,20 @@ class Gemma2PreTrainedModel(PreTrainedModel):
             if module.padding_idx is not None:
                 module.weight.data[module.padding_idx].zero_()
 
+    @classmethod
+    def _check_and_enable_sdpa(cls, config, hard_check_only: bool = False):
+        """
+        Overloads `PreTrainedModel._check_and_enable_sdpa` so as to DISABLE torch SDPA by default on Gemma2 models.
+        SDPA reduces the model performance on Gemma2 because of the logits softcapping.
+        """
+        config = super()._check_and_enable_sdpa(config, hard_check_only=hard_check_only)
+
+        # if using the default path -> swap sdpa by eager
+        if not hard_check_only and config._attn_implementation == "sdpa":
+            config._attn_implementation = "eager"
+
+        return config
+
 
 _CONFIG_FOR_DOC = "Gemma2Config"
 

tests/models/gemma2/test_modeling_gemma2.py

@@ -81,14 +81,31 @@ class Gemma2ModelTest(GemmaModelTest, unittest.TestCase):
         self.model_tester = Gemma2ModelTester(self)
         self.config_tester = ConfigTester(self, config_class=Gemma2Config, hidden_size=37)
 
-    @unittest.skip("Eager and SDPA do not produce the same outputs, thus this test fails")
+    @unittest.skip("Failing because of unique cache (HybridCache)")
     def test_model_outputs_equivalence(self, **kwargs):
         pass
 
-    @unittest.skip("Gemma2's outputs are expected to be different")
+    @unittest.skip("Gemma2's eager attn/sdpa attn outputs are expected to be different")
     def test_eager_matches_sdpa_inference(self):
         pass
 
+    @unittest.skip("Gemma2's eager attn/sdpa attn outputs are expected to be different")
+    def test_sdpa_equivalence(self):
+        pass
+
+    def test_eager_attention_loaded_by_default(self):
+        """Gemma 2 + SDPA = inferior results, because of the logit softcapping. Eager is the default."""
+        config, _ = self.model_tester.prepare_config_and_inputs_for_common()
+
+        # Usually we enable SDPA by default, but not for Gemma2
+        model = Gemma2Model(config)
+        self.assertTrue(model.config._attn_implementation == "eager")
+
+        # We can still force SDPA
+        config._attn_implementation = "sdpa"
+        model = Gemma2Model(config)
+        self.assertTrue(model.config._attn_implementation == "sdpa")
+
 
 @slow
 @require_torch_gpu