add modular pipeline for qwen-image 2.1

[akshan-main]

What does this PR do?

Adds a modular pipeline for Qwen-Image 2.1: QwenImage21AutoBlocks + QwenImage21ModularPipeline, covering text-to-image and image-conditioned generation with one or more condition images. Same shape as flux2 (text_encoder / vae_encoder / denoise / decode, with the image-conditioned variants selected on image and image_latents). The denoiser is shared by both workflows and keeps one prefix KV cache per guidance branch, so the cache also works when the guider starts or stops mid-run. sample_sigmas is a pipeline config, so the checkpoint's grid loads from model_index.json as in QwenImage21Pipeline.

Outputs match QwenImage21Pipeline exactly with shared components and seed, on the tiny components and on Qwen/Qwen-Image-2.1 in bf16 (text-to-image, edit, two-image edit, with and without guidance and the KV cache). Test components live in akshan-main/tiny-qwenimage21-modular-pipe.

Also registers QwenImage21Pipeline in the auto text-to-image mapping so ModularPipeline.from_pretrained("Qwen/Qwen-Image-2.1") resolves the modular class, and makes the test helper that reads modular_model_index.json skip null config values.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Discussed on Slack with @sayakpaul @yiyixuxu
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@yiyixuxu @sayakpaul

[github-actions]

Hi @akshan-main, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.

Please note that PRs without a linked issue are likely to be automatically closed 10 days after this notice.

Once the PR links an issue (or gets the no-issue-needed label), you can ignore this message — it stays here as a comment, but it no longer applies.