hao-ai-lab-FastVideo/v2

v2

A scoped, CPU-testable realization of the model-native, (recipe, runtime) runtime. The full write-up is ../designv4.md — the unified design, reflecting exactly what is built and tested here (with ../design_v3.md as the north-star argument it elaborates).

It is "mini" in two senses: (1) it supports a deliberate subset of models/methods, and (2) the heavy neural forwards are small numpy stand-ins so the whole thing runs and is tested on a laptop with no GPU and no torch. The architecture is real; the kernels are toys. Backends are selected through the platform/ dispatch substrate: two tuple-keyed registries — COMPONENTS(kind, device, variant) and KERNELS(op, device, arch, variant) — that a detected Platform resolves, with the numpy reference as the terminal fallback rung and parity oracle (§17). On a GPU box you register the torch component/kernel backends (the cuda cells are declared in platform/backends/torch_cuda.py, available=False until torch+CUDA are present) and Platform.detect() resolves them — the loops, scheduler, caches, parity, training, and workflow code are unchanged.

Honest scope of what is wired today (the substrate supports more than it demonstrates): a pure- python accel stand-in backend proves the dispatch is genuinely device-generic (cross-device resolution + arch fallback + the parity oracle, bit-identical to numpy) without a GPU. The wan21 denoise loop routes its solver ops (flow_match_step/flow_sde_step) through the kernel table, and the dit component has a backend override; the other model loops still call the numpy samplers directly and only the dit kind is overridden — each is a one-line change to adopt. The full torch/CUDA path and a cudagraph capture-safety (workspace_bytes) contract are declared/​deferred, not implemented.

183 tests, 30 files, run two ways (pytest and a zero-dependency runner). Python 3.10+ and numpy only.

Scope

	Supported
Inference	Wan2.1-1.3B (T2V, bidirectional diffusion) · LTX2.3 (two-stage distilled: 8-step base → upsample → 3-step refine) · Wan-causal (chunked/streaming rollout, the self-forcing student)
Training	finetuning (flow-match) · DMD2 distillation · DiffusionNFT (likelihood-free RL) · self-forcing (causal) — all on Wan2.1-1.3B
Joint / multi-expert RL	UniRL/PromptRL joint LM+generator RL (unified) · N-way joint RL over arbitrary experts (multi_expert) · end-to-end RL over a cross-model workflow (workflow_rl)
Omni / MoT	Cosmos3 & BAGEL — one resident MoT instance, ar_decode + diffusion_denoise on shared weights · Qwen-Omni thinker→talker→vocoder — three separate experts, three loop types, cascaded + streaming
Composition & scheduling	cross-model Workflow (T2I→I2V: flux-t2i → wan-i2v) · tiled VAE decode (VAE_TILE units co-scheduled with denoise steps)
Interactive	WorldModelSession — long-lived world-model rollout with persistent cross-request state, frame streaming, step-boundary cancellation

The one invariant (designv4 §1)

Model cards own components, loops, recipes, and parity.
Programs compose loops into tasks.   Workflows compose models into pipelines.
The scheduler executes the steps of loops as WorkUnits under one budget.
Caches are correct by key, not by hope.
Training records behavior on the same loops it serves.

Package layout → designv4 §12

Package	What it is	Validated by
card/	ModelCard = (components, loops, recipe, parity) as one versioned, validatable object; ModelInstance (one resident card; many loops bind shared components by reference)	test_contracts.py
loop/	the driven-loop contract init/next/advance/finalize; typed WorkPlan/StepResult/LoopState; the single LoopRunner; policies (CFG/flow-shift/precision/expert); the flow-match + FlowGRPO SDE samplers	test_policies.py, test_inference.py
program/	Program of typed nodes (ModelLoopNode/ComponentNode) for one model; Workflow + WorkflowRegistry for cross-model pipelines	test_inference.py, test_t2i_i2v_workflow.py
runtime/	Engine (serial + step-interleaved, workflow-aware), AsyncEngine, DisaggregatedRunner + role pools, layered scheduler, RuntimeLoopContext, WorldModelSession	test_admission.py, test_interleave_gate.py, test_serving.py, test_world_session.py
cache/	typed CacheKey (partitioned per-component by adapter+weights), per-class pools (feature/residual/slab_kv/paged_kv)	test_cache.py
memory/ · transport/ · parallel/	tagged pools + reservation-before-admission; connectors (chunk_ready + credit flow); ParallelPlan + pre-flight validation	test_admission.py, test_serving.py, test_contracts.py
parity/	ParityAligner, the C0–C4 consistency ladder, the batch-of-N interleave gate	test_parity.py, test_interleave_gate.py
extend/	observers (Profiler, NaNWatch) + interceptors (cache-dit ResidualSkipInterceptor), state per request & per CFG branch	test_extensions.py
training/	rollout = serve loop + capture; BehaviorRecord; WeightSyncPlan (per-component role); rewards; 7 methods (4 base + unified_rl, joint_multi_rl, workflow_rl)	test_training.py, test_unified_rl.py, test_joint_multi_rl.py, test_workflow_rl.py
serving/ · deploy/	our own OpenAI server (stdlib asyncio); DeploymentCard; our own LocalFleet; DynamoWorkerAdapter (frontable, not relied upon)	test_serving.py, test_serving_fixes.py
models/	the concrete cards: wan21/ ltx2/ wan_causal/ · omni/ cosmos3/ bagel/ qwen_omni/ · unified/ multi_expert/ · image_video/ tiled/ + the numpy toy backend	all

Enforced boundary (designv4 §3, §8): training/ imports card/loop/runtime/models; the engine never imports training. card/ imports no runtime. Cross-model Workflow orchestration sits above the engine (no change to the single-instance hot path).

What is demonstrated (each backed by a test)

Core contracts:

• Loop inversion is safe under interleaving — concurrent requests interleaved at step granularity are bit-identical to serial, across every model and loop type (test_interleave_gate.py, test_omni.py). A buggy module-global interceptor breaks the gate; the per-request one passes.
• The (recipe, runtime) pair is enforced — a card whose recipe assumes_loop an undeclared loop fails validate() (test_contracts.py).
• Caches are correct by key, partitioned per component, so a transformer weight-sync does not flush a frozen text-encoder's feature cache (test_cache.py).
• Train ≡ serve by construction — every method drives the same loop the engine serves, in the ROLLOUT profile (test_training.py).

Stress tests (the design held — see designv4 §9):

• §9.3 Joint LM+generator RL (UniRL/PromptRL) — one reward → a token policy-gradient on the LM and a FlowGRPO PPO on the DiT; likelihood-based C2 (per-step log-prob identity ⇒ PPO ratio == 1).
• §9.5 Qwen-Omni cascade — three separate experts, three loop types, cross-stage conditioning, streaming codec→waveform.
• §9.6 Cross-model Workflow — T2I→I2V across two distinct models; the I2V stage provably consumes the generated image; each model keeps its own interleave-parity guarantee.
• §9.7 N-way joint RL — generalizes joint RL to N experts (per-component sync, dict grad-targets); the substrate was already N-ready, only the method body looped over two.
• §9.8 Interactive world-model session — persistent cross-request state, transactional step-boundary cancellation, no cross-session smearing (test_world_session.py).
• §9.9 End-to-end RL over a workflow — one final-video reward trains an earlier model (the T2I generator); proven causal by a control (constant reward ⇒ nothing moves) (test_workflow_rl.py).
• §9.10 Heterogeneous WorkUnit co-scheduling — VAE_TILE units interleave bit-identically with DIFFUSION_STEP units through one budget; tiling is exact (test_tiled_scheduling.py).

Run it

cd /Users/willlin/src/FastVideo-mini
python3 -m pytest v2/tests/ -q      # 127 tests
python3 v2/run_tests.py             # same suite, ZERO deps (no pytest needed)
python3 -m v2.examples              # the worked examples

Running the real models (GPU)

Each ComponentSpec records the real load_id (e.g. fastvideo.models.dits.wanvideo:WanTransformer3DModel). To run real Wan/LTX/etc.:

1. install torch + the parent fastvideo package + weights;
2. implement the cuda (or other-device) cells in platform/backends/ — a component builder that loads the real module and exposes the same call surface the loops use (a velocity forward; vae.decode; text_encoder.encode), and the device kernels for the solver/primitive ops. They flip to available=True when torch+CUDA are present, and Platform.detect() resolves them; the numpy ComponentSpec.factory stays as the CPU terminal fallback rung;
3. for the loops that don't yet route through the kernel table, swap the direct numpy flow-match/SDE sampler calls for model.platform.kernels.get(...) (the wan21 loop shows the pattern — a one-line change per call site). The loops, policies, scheduler, caches, parity gates, training methods, and workflows are unchanged.

Serving & fleet (our own version — Dynamo is an option, not a dependency)

AsyncEngine (queue, lifecycle, live streaming, step-boundary cancellation) · role/stage pools + disaggregation (output bit-identical to inline) · transport/ connectors (chunk_ready + credit flow) · our own LocalFleet (cost/affinity/least-loaded routing, health/drain) · a DynamoWorkerAdapter exporting the same DeploymentCard + cost model so Dynamo can front us · a framework-free OpenAI server (/v1/chat SSE, /v1/images, /v1/videos job+poll, /v1/models, /health, /metrics). Workflows are first-class servables — a workflow_id is addressable in the same namespace as a model_id (see designv4 §9.6).

import asyncio
from v2.models import build_default_engine, build_omni_engine
from v2.runtime import AsyncEngine
from v2.serving import OmniOpenAIServer
async def serve():
    eng = build_default_engine(); build_omni_engine(eng)
    host, port = await OmniOpenAIServer(AsyncEngine(eng)).serve(port=8000)
asyncio.run(serve())

Deliberately out of scope

The ComfyUI workflow compiler, WebRTC realtime transport (the interactive session logic is built; the realtime wire is not), full LLM-grade AR serving (radix prefix trees, speculative decoding), and real distributed parallelism (the mesh builder is a CPU fake; multi-node is multiple pools fronted by the fleet). A live Dynamo integration is also out — we ship the adapter + contract, and LocalFleet covers the fleet role without it. The honest open questions (does step-level scheduling pay; the <100ms interactive latency target; quality/C4) are GPU-port measurements, named in designv4 §13.