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