[generate] return Cache object even if passed in a legacy format (#35673)

* generate returns a Cache object by default

* fix tests

* fix test for encoder-decoder models

tests/models/gemma2/test_modeling_gemma2.py

@@ -117,11 +117,6 @@ class Gemma2ModelTest(GemmaModelTest, unittest.TestCase):
     def test_dola_decoding_sample(self):
         pass
 
-    @parameterized.expand([(1, False), (1, True), (4, False)])
-    @unittest.skip("Gemma2 has HybridCache and doesn't support old tuple format at all")
-    def test_new_cache_format(self, num_beams, do_sample):
-        pass
-
     @unittest.skip("Gemma2 has HybridCache and doesn't support continue from past kv")
     def test_generate_continue_from_past_key_values(self):
         pass