Generation: deprecate PreTrainedModel inheriting from GenerationMixin (#33203)

tests/generation/test_utils.py

@@ -2099,6 +2099,15 @@ class GenerationTesterMixin:
             )
             self.assertEqual(with_all_logits.tolist(), without_all_logits.tolist())
 
+    @pytest.mark.generate
+    def test_inherits_generation_mixin(self):
+        """
+        Tests that the model class directly inherits `GenerationMixin`, as opposed to relying on `PreTrainedModel`
+        to inherit it.
+        """
+        for model_class in self.all_generative_model_classes:
+            self.assertTrue("GenerationMixin" in str(model_class.__bases__))
+
     def _check_outputs(self, output, input_ids, config, use_cache=False, num_return_sequences=1):
         batch_size, seq_length = input_ids.shape
         config = config.text_config if hasattr(config, "text_config") else config