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. The Turbo checkpoint maps to its own blockset (QwenImage21TurboAutoBlocks / QwenImage21TurboModularPipeline) in the flux2 klein shape: no guider, no negative_prompt, and the sample_sigmas grid it ships with, which is also what ModularPipeline.from_pretrained routes on.

Outputs match QwenImage21Pipeline exactly with shared components and seed, on the tiny components and on Qwen/Qwen-Image-2.1 and Qwen/Qwen-Image-2.1-Turbo 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 and akshan-main/tiny-qwenimage21-turbo-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.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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@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.

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[yiyixuxu]

thanks!
sorry i'm a bit late on review, really nice PR!
left some comments

[yiyixuxu]

can we remove guider as a component for tex encoder bock?
(a lot of our current text_encoder lists guider as a components, but i write a bit on the preferred pattern here #14469)

[akshan-main]

removed from both text encoder blocks. On their own they encode a negative_prompt when given; next to the denoiser they follow the guider and warn when one is dropped.

[yiyixuxu]

ohh I think this is only for distilled checkpoint, actually can we seperate the blockset for distilled checkpiont
code is in this PR #14950
repo https://huggingface.co/Qwen/Qwen-Image-2.1-Turbo

flux2 is good reference https://github.com/huggingface/diffusers/tree/main/src/diffusers/modular_pipelines/flux2

[HuggingFaceDocBuilderDev]

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