Add Model Port Encoder

Goal

Prototype or parity-debug one encoder or encoder-like conditioner in FastVideo-native code. Use this for text encoders, image encoders, audio encoders, and compound conditioners that fit the encoder config/loader bucket.

Inputs

Follow ../add-model/shared/component_skill_common.md and require the complete packet from ../add-model/contracts/component_context.md.

Encoder-specific packet fields:

• component: encoder or encoder-like conditioner name.
• parity_test: tests/local_tests/encoders/test_<family>_<component>_parity.py.
• weights: converted encoder dir, HF subfolder, or external HF id.
• target_files: fastvideo/models/encoders/<arch_or_family>.py and fastvideo/configs/models/encoders/<arch_or_family>.py.

Modes

Use the common prototype and parity-debug modes from ../add-model/shared/component_skill_common.md.

Encoder-specific prototype concerns include tokenizer kwargs, hidden-state extraction, output packing, connector order, and external/passthrough weight needs.

Reuse Proof

Apply the shared reuse proof. Encoder-specific comparison must include tokenizer contracts, hidden-state extraction, masks, positional IDs, output packing, connector/projection ordering, passthrough paths, and returned dataclass shape.

Existing FastVideo Patterns

• Base classes: TextEncoder and ImageEncoder in fastvideo/models/encoders/base.py.
• Output type: BaseEncoderOutput.
• Config bases: TextEncoderConfig, ImageEncoderConfig, TextEncoderArchConfig, and ImageEncoderArchConfig in fastvideo/configs/models/encoders/base.py.
• Use the matching encoder config bucket. Wrong bucket inheritance can typecheck but fail during pipeline wiring.
• Config export: add the config to fastvideo/configs/models/encoders/__init__.py.
• Registry discovery: set EntryClass = <ClassName> or a list of class names in the model file.
• Reference examples: native t5.py, clip.py, siglip.py, llama.py, qwen2_5.py, gemma.py, and compound stable_audio_conditioner.py.
• Layer guidance: fastvideo/layers/AGENTS.md.

Implementation Rules

• Reuse tokenizers and pure data utilities when needed, but do not add runtime third-party model-class imports as a placeholder for a component that owns weights or numerical behavior.
• For LLM-style encoders, follow existing tensor-parallel patterns such as QKVParallelLinear, MergedColumnParallelLinear, RowParallelLinear, VocabParallelEmbedding, and RMSNorm when matching native examples.
• Match official hidden-state extraction exactly: layer index, pooled output, attention mask dtype, padding side, truncation, special tokens, final norm, output_hidden_states, and returned tuple/dataclass shape.
• For connector or conditioner modules, preserve sub-conditioner order and the exact packing of cross-attention tokens, masks, and global conditioning.
• Put tokenizer kwargs and architecture constants on the arch config when they affect numerical behavior.
• If an external HF encoder is explicitly accepted as a lazy wrapper, keep it isolated, document why it is not a native port, and still require parity for the wrapper's output contract.

Hybrid external-HF encoder checklist:

• Put external model folders in passthrough subfolders such as text_encoder/<external_name>/, or record a root model_index.json path field that the loader resolves to a local directory.
• Keep external model parameters out of the FastVideo-owned state-dict surface when the external model is loaded lazily from its own HF files.
• Convert and strict-check only the FastVideo-owned connector/projection weights; document external model weights as passthrough.
• Add parity for the wrapper's final output contract and, when useful, a narrower connector-only parity test that labels its scope as implementation_subcomponent.
• Verify the production loader resolves the same external path used by the pipeline, not just the direct class used in the parity test.

Prototype Checks

Follow the shared prototype success criteria.

Parity-Debug Loop

Run the shared parity-debug loop. The component test command is:

pytest <parity_test> -v -s

For numerical drift, check tokenization, masks, hidden-state selection, positional IDs, dtype/autocast, and output packing before changing layers.

Escape Hatches

Follow ../add-model/shared/common_rules.md and the component-specific guidance in ../add-model/shared/component_skill_common.md. Encoder-specific ask cases include accepting private model-code execution, choosing between incompatible tokenizer/encoder references, or dropping a required conditioning stream.

Handoff

Return ../add-model/contracts/component_skill_handoff.md following the common handoff rules in ../add-model/shared/component_skill_common.md.