Added support of QEffDiffusionPipeline for Diffusers

[quic-amitraj]

Summary

Adds a generic QEffDiffusionPipeline entrypoint for diffusers that auto-dispatches to the correct concrete QEff pipeline, while moving shared infra to a common base.

What Changed

• Added QEffDiffusionPipeline.from_pretrained(...) dispatch by HF pipeline class and return of concrete child objects:
  • QEffFluxPipeline
  • QEffWanPipeline
  • QEffWanImageToVideoPipeline
• Centralized shared compile/export flow in generic base APIs.
• Refactored child pipelines to inherit and reuse the shared infra.
• Updated all diffusion example scripts (including custom + first-block-cache) to use generic factory creation.
• Added consolidated unit tests in tests/unit_test/utils/test_diffusion_pipeline_infra.py.