mflux model porting

Goal

Provide a repeatable, MLX-focused workflow for porting ML models (typically from diffusers repo located near mflux repo in the system) into mflux with correctness first, then refactor to mflux style.

Principles

• Match the reference implementation first; prove correctness before cleanup.
• Lock correctness with deterministic tests before refactoring.
• During the initial port, avoid premature performance work (e.g., mx.compile, kernel fusion tweaks, scheduler micro-optimizations); add optimizations only after correctness is locked.
• Refactor toward shared components and clean APIs once tests are green.
• PyTorch and MLX RNGs are different; for strict parity checks, export the exact initial noise/latents from the reference and load them in MLX instead of relying on matching integer seeds.

Workflow (checklist)

1. Scope and parity
   • Define target parity (outputs, speed, memory) and acceptable tolerances.
   • Identify reference files, configs, and checkpoints to mirror.
   • Draft a Cursor plan for the port and review it before starting implementation.
2. Port fast to reference
   • Add the model package skeleton and a variant class + initializer.
   • Follow standard mflux initializer/weight-loading style; review recent ports like z_image_turbo and flux2_klein for structure and naming.
   • Wire weight definitions/mappings early so loading is exercised (implement quantization in the initializer, but skip it during early runs).
   • Keep the first implementation simple and explicit; defer mx.compile and other speed-focused changes until deterministic parity is passing.
   • When defining explicit weight mappings, inspect actual tensor values from the model in the Hugging Face cache to confirm names and shapes.
   • Add a minimal hardcoded runner for quick iteration (two tiny scripts: one in the reference repo, one in mflux), seeded with diffusers-style defaults (e.g., 1024×1024, default prompt).
   • Add lightweight shape checks close to the code paths.
   • Use mx.save/mx.load at critical points; it is OK to add these to the reference (without changing logic) to export latents.
3. Port order (work backwards from image)
   • Typical image generation flow: prompt → text_encoder → transformer_loop → VAE → image.
   • For porting, invert the order so you can validate pixel space early.
   • Start with VAE decode/encode to validate output images quickly:
     • Export packed latents from the reference just before VAE decode.
     • Load latents inline and decode to an image for visual inspection.
     • Run an encode→decode roundtrip to sanity check reconstruction; a good-looking image reconstruction increases confidence in the implementation.
     • Expect small numeric diffs in tensor values; when it is not clear from the numbers alone, always generate images and rely on human visual inspection to judge whether the match is acceptable.
   • Then port the transformer loop and its schedulers with intermediate latent checks.
     • If the reference uses a novel scheduler, port it; otherwise, reuse the existing mflux scheduler.
   • Finish with the text encoder and tokenizer details.
   • After each major component is validated (e.g., VAE, transformer, text encoder), commit with a clear milestone message like "VAE done" to preserve progress.
   • Once the full port is working, remove any loaded tensors or debug artifacts so no traces remain.
4. Deterministic validation
   • Create a deterministic MLX test (image or tensor) that locks the output.
   • Run tests via MFLUX_PRESERVE_TEST_OUTPUT=1 uv run <test command>.
   • If MLX OOMs on sensible inputs (e.g., 1024×1024), assume a likely porting mistake and re-check shapes or memory-heavy ops.
5. Post-test refactor (explicit step)
   • Review commits after the first deterministic test to capture refactoring preferences.
   • Consolidate shared components into common modules.
   • Remove debug paths and one-off schedulers once validated.
   • Move configuration defaults into standard config/scheduler paths.
   • Simplify and decompose large files into focused modules once behavior is locked.
   • Prefer shared scheduler implementations when they already exist in mflux.
   • Ensure CLIs register callbacks via CallbackManager.register_callbacks(...) so shared features like --stepwise-image-output-dir work; pass a latent_creator that supports unpack_latents(...).
   • Keep running the deterministic image test during refactors to avoid regressions.
6. Finalize
   • Re-run tests and basic perf checks.
   • Add CLI/pipeline defaults and completions later, once core output is stable.
   • Ensure the model is wired into the standard surfaces:
     • ModelConfig entry + aliases
     • Thin model CLI entrypoint that uses shared parser/config/callback patterns
     • README following the structure and tone of existing model READMEs
     • Python API example that matches the CLI/defaults
   • Document any new mapping rules, shape constraints, or tolerances.

Tooling expectations

• Use uv for running scripts and tests: uv run <command>.
• Prefer uv run python -m <module> for local modules.

Deliverables

• Deterministic MLX test that verifies correctness.
• Documented weight mapping, shape constraints, and any known tolerances.
• Cleaned, shared components aligned with mflux style.
• Standard mflux surfaces in place: config aliases, thin CLI, and a README/examples pass aligned with the final behavior.