FlaxStableDiffusionImg2ImgPipeline should delete prepare_inputs and provide prepare_text_inputs and prepare_image_inputs

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...].
in https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/stable_diffusion/pipeline_flax_stable_diffusion_img2img.py#L170
prepare_inputs(self, prompt: Union[str, List[str]], image: Union[Image.Image, List[Image.Image]]) accept both prompt and image
But there are prompt_ids and neg_prompt_ids in https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/stable_diffusion/pipeline_flax_stable_diffusion_img2img.py#L338 which prompt (positive or negative) I should put to prepare_inputs?

**Describe the solution you'd like.**
A clear and concise description of what you want to happen.
like https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/controlnet/pipeline_flax_controlnet.py#L180 and https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/controlnet/pipeline_flax_controlnet.py#L194
delete prepare_inputs and provide prepare_text_inputs and prepare_image_inputs

**Describe alternatives you've considered.**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context.**
Add any other context or screenshots about the feature request here.

Start in src/diffusers/pipelines/stable_diffusion/pipeline_flax_stable_diffusion_img2img.py, especially prepare_inputs and the prompt_ids and neg_prompt_ids usage around the cited lines. Compare the separate prepare_text_inputs and prepare_image_inputs methods in src/diffusers/pipelines/controlnet/pipeline_flax_controlnet.py. Done means the img2img pipeline no longer exposes prepare_inputs and provides the two separate preparation methods without ambiguity about positive and negative prompts.

Scritto dal modello di indicizzazione a partire dal testo della issue.