Iterative generation using Input embeds and past_key_values (#35890)

* Iterative generation using input embeds

* ruff fix

* Added Testcase

* Updated comment

* ♻️ Refactored testcase

* Skip test for these models

* Continue generation using input embeds and cache

* Skip generate_continue_from_embeds test

* Refactor `prepare_input_for_generation` func

* Continue generation using input embeds and cache

* Modular changes fix

* Overwrite 'prepare_inputs_for_generation' function

tests/models/zamba2/test_modeling_zamba2.py

@@ -333,6 +333,10 @@ class Zamba2ModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMix
         """
         pass
 
+    @unittest.skip(reason="Zamba2 has hybrid cache.")
+    def test_generate_continue_from_inputs_embeds(self):
+        pass
+
     @unittest.skip(reason="A large mamba2 would be necessary (and costly) for that")
     def test_multi_gpu_data_parallel_forward(self):
         pass