Modular Pipeline: support for PixArtAlphaPipeline

Model/Pipeline/Scheduler description

I would like to implement a modular version of the PixArt pipeline family, as discussed in #13295.

The initial scope includes the PixArtAlphaPipeline under src/diffusers/pipelines/pixart_alpha/. Currently, the focus is on PixArt-α only. The implementation would be structured so that PixArtSigmaPipeline support can be added later in a follow-up.

Current plan:

Add a new modular PixArt package under src/diffusers/modular_pipelines/, following the structure of the existing built-in modular pipelines.

• encoders for prompt encoding
• before_denoise for input / latent preparation and timestep setup
• denoise for the main PixArt denoising loop
• decoders for VAE decoding and post-processing

I’m happy to implement this myself, and this would be my first contribution to Diffusers.

Open source status

• The model implementation is available.
• The model weights are available (Only relevant if addition is not a scheduler).

Provide useful links for the implementation

Relevant references:

• Current Diffusers implementation:
  • src/diffusers/pipelines/pixart_alpha/pipeline_pixart_alpha.py
  • src/diffusers/pipelines/pixart_alpha/pipeline_pixart_sigma.py
• PixArt docs / weights:
  • https://huggingface.co/docs/diffusers/api/pipelines/pixart
  • https://huggingface.co/PixArt-alpha/PixArt-XL-2-1024-MS

Bắt đầu với src/diffusers/pipelines/pixart_alpha/pipeline_pixart_alpha.py và xem xét cấu trúc modular pipeline tích hợp hiện có. Tổ chức một package mới dưới src/diffusers/modular_pipelines/ với encoders, before_denoise, denoise và decoders cho PixArt-α. Hoàn thành khi PixArtAlphaPipeline có hỗ trợ modular, còn PixArtSigmaPipeline vẫn nằm ngoài phạm vi.