[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/bamba/test_modeling_bamba.py

@@ -18,7 +18,6 @@ import inspect
 import unittest
 
 import pytest
-from parameterized import parameterized
 
 from transformers import AutoTokenizer, BambaConfig, is_torch_available
 from transformers.testing_utils import (
@@ -395,11 +394,6 @@ class BambaModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixi
                 [self.model_tester.num_attention_heads, encoder_seq_length, encoder_key_length],
             )
 
-    @unittest.skip(reason="Bamba 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
-
     def test_batching_equivalence(self):
         # need to disable the tril input mask
         orig = self.model_tester.use_input_mask