Pass input_ids to _update_model_kwargs_for_generation

### Feature request For flexibility in adding additional kwargs to models and handling them consistently with generation utils, it might be useful if input_ids was available in _update_model_kwargs_for_generation. ### Motivation My current use-case involves passing a user-defined position index, separate from position_ids, and incrementing it during generation depending on which tokens are generated (e.g. if a sep token is generated, the position index resets, whereas if a standard token is generated, it increments by 1). The most natural way for me to handle generation in such a model would be to overwrite _update_model_kwargs_for_generation and handle the incrementation index there. However, this would require access to input_ids. Although the logic can be handled by prepare_inputs_for_generation, because this cannot update model_kwargs, it cannot be computed incrementally, making the computation more complicated. I expect other custom use cases would also benefit from this change. ### Your contribution If this sounds reasonable it is a simple change which I'd happily help with