Transformers documentation

Fusion mapping (experimental feature)

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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:

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

class transformers.fusion_mapping.ModuleFusionSpec

< >

( )

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_log

< >

( model_name: str )

Return the log message emitted when no compatible modules are found.

is_fusable

< >

( module: Module )

Return whether module is compatible with this fusion family.

make_fused_class

< >

( original_cls: type )

Build the runtime replacement class for a compatible module class.

make_transforms

< >

( config: PretrainedConfig )

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

class transformers.fusion_mapping.PatchEmbeddingsFusionSpec

< >

( )

Fuse compatible Conv3d patch embeddings into flattened Linear projections.

transformers.fusion_mapping._register_module_fusion

< >

( config: PretrainedConfig fusion_name: str spec: ModuleFusionSpec )

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

< >

( config fusion_config: collections.abc.Mapping[str, bool | collections.abc.Mapping[str, typing.Any]] | None = None )

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
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