transformers/pr_41992/en/fusion_mapping.md

Fusion mapping (experimental feature)

Fusion mapping provides an opt-in way to replace model submodules at load time while preserving the original checkpoint format.

It builds on:

• Monkey patching to swap module classes before model instantiation.
• Dynamic weight loading to map weights between the original and fused runtime layouts.

Fusion mapping is an experimental loading feature. It changes the runtime module structure and may affect model behavior. Use it only when you explicitly want a fused runtime layout.

Quick start

Fusion is enabled through from_pretrained() with fusion_config:

from transformers import AutoModelForImageTextToText

model = AutoModelForImageTextToText.from_pretrained(
    "Qwen/Qwen2-VL-2B-Instruct",
    fusion_config={"patch_embeddings": True},
)

By default, no fusion is applied. If fusion_config is stored in the model config, from_pretrained() will reuse it automatically.

How it works

Fusion registration happens before the model is instantiated:

1. from_pretrained() uses the explicit fusion_config argument or falls back to config.fusion_config.
2. The fusion registry validates the requested fusion names.
3. Each enabled fusion meta-initializes the target model class, optionally filters candidate modules by name, and uses is_fusable(...) to discover compatible module classes.
4. Fused replacement classes are registered through register_patch_mapping().
5. Matching ~WeightTransform rules are generated from the config so checkpoint loading can map weights into the fused runtime layout.
6. By default, save_pretrained() uses the reverse conversion path to restore the original checkpoint layout. Pass save_original_format=False to keep the converted runtime layout instead.

This lets a fusion use a different runtime module structure while still loading from the original checkpoint format, and by default saving back to it as well.

Note: With the current monkey-patching mechanism, fusion registration is class-level: one compatible module class maps to one fused replacement class.

Current fusion families

Currently, fusion_config supports one fusion family:

• patch_embeddings Enable with:

  fusion_config = {"patch_embeddings": True}
  

  Effect: Replaces compatible nn.Conv3d patch embedding projections with equivalent flattened nn.Linear projections at runtime.

Extending fusion mapping

To add a new fusion family:

1. Add an is_fusable predicate. This decides whether a discovered module is compatible with the fusion.
2. Optionally add target_modules_patterns. This makes the discovery step more explicit by pre-filtering candidate module names before is_fusable(...).
3. Add a make_fused_class factory. This returns the runtime replacement class for a compatible module class.
4. Add a make_transforms factory if the fused layout needs checkpoint conversion. This returns the ~WeightTransform rules that map weights between the original and fused layouts for a given config.
5. Register the new ModuleFusionSpec in fusion_mapping.py.

Once registered, the new fusion becomes available through fusion_config.

Internal API[[transformers.fusion_mapping.ModuleFusionSpec]]

transformers.fusion_mapping.ModuleFusionSpec[[transformers.fusion_mapping.ModuleFusionSpec]]

Source

Base recipe for a fusion family.

A fusion spec decides which modules are eligible for a fusion, how to build the runtime replacement class, and which weight transforms are needed to map checkpoints between the original and fused layouts.

get_empty_logtransformers.fusion_mapping.ModuleFusionSpec.get_empty_loghttps://github.com/huggingface/transformers/blob/vr_41992/src/transformers/fusion_mapping.py#L54[{"name": "model_name", "val": ": str"}] Return the log message emitted when no compatible modules are found.

is_fusable[[transformers.fusion_mapping.ModuleFusionSpec.is_fusable]]

Source

Return whether module is compatible with this fusion family.

make_fused_class[[transformers.fusion_mapping.ModuleFusionSpec.make_fused_class]]

Source

Build the runtime replacement class for a compatible module class.

make_transforms[[transformers.fusion_mapping.ModuleFusionSpec.make_transforms]]

Source

Build the weight transforms needed to load and save the fused runtime layout.

transformers.fusion_mapping.PatchEmbeddingsFusionSpec[[transformers.fusion_mapping.PatchEmbeddingsFusionSpec]]

Source

Fuse compatible Conv3d patch embeddings into flattened Linear projections.

transformers.fusion_mapping._register_module_fusion[[transformers.fusion_mapping._register_module_fusion]]

Source

Register one fusion family for cls.

This function updates the two global registries used by fused loading:

• the monkey-patching registry, so compatible module classes are replaced before initialization
• the checkpoint conversion mapping, so fused runtime modules still load from the original checkpoint layout

Notes:

• conflicting checkpoint transforms fail fast

transformers.fusion_mapping.register_fusion_patches[[transformers.fusion_mapping.register_fusion_patches]]

Source

Register requested runtime fusions for cls.

This function:

• validates fusion_config against _FUSION_REGISTRY
• resolves the enabled fusion families in user order
• registers monkey patches and checkpoint transforms before model instantiation