GitLab

* Add ZImageImg2ImgPipeline

Updated the pipeline structure to include ZImageImg2ImgPipeline
    alongside ZImagePipeline.
Implemented the ZImageImg2ImgPipeline class for image-to-image
    transformations, including necessary methods for
    encoding prompts, preparing latents, and denoising.
Enhanced the auto_pipeline to map the new ZImageImg2ImgPipeline
    for image generation tasks.
Added unit tests for ZImageImg2ImgPipeline to ensure
    functionality and performance.
Updated dummy objects to include ZImageImg2ImgPipeline for
    testing purposes.

* Address review comments for ZImageImg2ImgPipeline

- Add `# Copied from` annotations to encode_prompt and _encode_prompt
- Add ZImagePipeline to auto_pipeline.py for AutoPipeline support

* Add ZImage pipeline documentation

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Co-authored-by: YiYi Xu <yixu310@gmail.com>
Co-authored-by: Álvaro Somoza <asomoza@users.noreply.github.com>