[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/zamba/test_modeling_zamba.py

@@ -19,7 +19,6 @@ import tempfile
 import unittest
 
 import pytest
-from parameterized import parameterized
 
 from transformers import AutoTokenizer, ZambaConfig, is_torch_available
 from transformers.testing_utils import (
@@ -551,11 +550,6 @@ class ZambaModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixi
         """
         self.skipTest(reason="Zamba flash attention does not support right padding")
 
-    @unittest.skip(reason="Zamba has its own special cache type")
-    @parameterized.expand([(1, False), (1, True), (4, False)])
-    def test_new_cache_format(self, num_beams, do_sample):
-        pass
-
 
 @require_torch
 class ZambaModelIntegrationTest(unittest.TestCase):