Add support for export comfyui compatible checkpoint for diffusion model(e.g., LTX-2)

Signed-off-by: ynankani <ynankani@nvidia.com>

Author: ynankani

modelopt/torch/export/unified_export_hf.py

@@ -28,7 +28,7 @@
 
 import torch
 import torch.nn as nn
-from safetensors.torch import save_file
+from safetensors.torch import save_file, load_file, safe_open
 
 try:
     import diffusers
@@ -111,20 +111,108 @@ def _is_enabled_quantizer(quantizer):
     return False
 
 
+def _merge_diffusion_transformer_with_non_transformer_components(
+    diffusion_transformer_state_dict: dict[str, torch.Tensor],
+    merged_base_safetensor_path: str,
+) -> tuple[dict[str, torch.Tensor], dict[str, str]]:
+    """Merge diffusion transformer weights with non-transformer components from a safetensors file.
+
+    Non-transformer components (VAE, vocoder, text encoders) and embeddings connectors are
+    taken from the base checkpoint. Transformer keys are prefixed with 'model.diffusion_model.'
+    for ComfyUI compatibility.
+
+    Args:
+        diffusion_transformer_state_dict: The diffusion transformer state dict (already on CPU).
+        merged_base_safetensor_path: Path to the full base model safetensors file containing
+            all components (transformer, VAE, vocoder, etc.).
+
+    Returns:
+        Tuple of (merged_state_dict, base_metadata) where base_metadata is the original
+        safetensors metadata from the base checkpoint.
+    """
+
+    base_state = load_file(merged_base_safetensor_path)
+
+    non_transformer_prefixes = [
+        'vae.', 'audio_vae.', 'vocoder.', 'text_embedding_projection.',
+        'text_encoders.', 'first_stage_model.', 'cond_stage_model.', 'conditioner.',
+    ]
+    correct_prefix = 'model.diffusion_model.'
+    strip_prefixes = ['diffusion_model.', 'transformer.', '_orig_mod.', 'model.', 'velocity_model.']
+
+    base_non_transformer = {k: v for k, v in base_state.items()
+                            if any(k.startswith(p) for p in non_transformer_prefixes)}
+    base_connectors = {k: v for k, v in base_state.items()
+                       if 'embeddings_connector' in k and k.startswith(correct_prefix)}
+
+    prefixed = {}
+    for k, v in diffusion_transformer_state_dict.items():
+        clean_k = k
+        for prefix in strip_prefixes:
+            if clean_k.startswith(prefix):
+                clean_k = clean_k[len(prefix):]
+                break
+        prefixed[f"{correct_prefix}{clean_k}"] = v
+
+    merged = dict(base_non_transformer)
+    merged.update(base_connectors)
+    merged.update(prefixed)
+    with safe_open(merged_base_safetensor_path, framework="pt", device="cpu") as f:
+        base_metadata = f.metadata() or {}
+
+    del base_state
+    return merged, base_metadata
+
+
 def _save_component_state_dict_safetensors(
-    component: nn.Module, component_export_dir: Path
+    component: nn.Module, 
+    component_export_dir: Path, 
+    merged_base_safetensor_path: str | None = None, 
+    hf_quant_config: dict | None = None
 ) -> None:
+    """Save component state dict as safetensors with optional base checkpoint merge.
+
+    Args:
+        component: The nn.Module to save.
+        component_export_dir: Directory to save model.safetensors and config.json.
+        merged_base_safetensor_path: If provided, merge with non-transformer components
+            from this base safetensors file.
+        hf_quant_config: If provided, embed quantization config in safetensors metadata
+            and per-layer _quantization_metadata for ComfyUI.
+    """
     cpu_state_dict = {k: v.detach().contiguous().cpu() for k, v in component.state_dict().items()}
-    save_file(cpu_state_dict, str(component_export_dir / "model.safetensors"))
-    with open(component_export_dir / "config.json", "w") as f:
-        json.dump(
-            {
-                "_class_name": type(component).__name__,
-                "_export_format": "safetensors_state_dict",
-            },
-            f,
-            indent=4,
+    metadata: dict[str, str] = {}
+    metadata_full: dict[str, str] = {}
+    if merged_base_safetensor_path is not None:
+        cpu_state_dict, metadata_full = _merge_diffusion_transformer_with_non_transformer_components(
+            cpu_state_dict,  merged_base_safetensor_path
         )
+    metadata["_export_format"] = "safetensors_state_dict" 
+    metadata["_class_name"] = type(component).__name__
+
+    if hf_quant_config is not None:
+        metadata_full["quantization_config"] = json.dumps(hf_quant_config)
+
+        # Build per-layer _quantization_metadata for ComfyUI
+        quant_algo = hf_quant_config.get("quant_algo", "unknown").lower()
+        layer_metadata = {}
+        for k in cpu_state_dict:
+            if k.endswith(".weight_scale") or k.endswith(".weight_scale_2"):
+                layer_name = k.rsplit(".", 1)[0]
+                if layer_name.endswith(".weight"):
+                    layer_name = layer_name.rsplit(".", 1)[0]
+                if layer_name not in layer_metadata:
+                    layer_metadata[layer_name] = {"format": quant_algo}
+        metadata_full["_quantization_metadata"] = json.dumps({
+            "format_version": "1.0",
+            "layers": layer_metadata,
+        })
+
+    metadata_full.update(metadata)
+    save_file(cpu_state_dict, str(component_export_dir / "model.safetensors"), metadata=metadata_full if merged_base_safetensor_path is not None else None)
+    
+    with open(component_export_dir / "config.json", "w") as f:
+        json.dump(metadata, f, indent=4)
 
 
 def _collect_shared_input_modules(
@@ -807,6 +895,7 @@ def _export_diffusers_checkpoint(
     dtype: torch.dtype | None,
     export_dir: Path,
     components: list[str] | None,
+    merged_base_safetensor_path: str | None = None,
     max_shard_size: int | str = "10GB",
 ) -> None:
     """Internal: Export diffusion(-like) model/pipeline checkpoint.
@@ -821,6 +910,8 @@ def _export_diffusers_checkpoint(
         export_dir: The directory to save the exported checkpoint.
         components: Optional list of component names to export. Only used for pipelines.
             If None, all components are exported.
+        merged_base_safetensor_path: If provided, merge the exported transformer with
+            non-transformer components from this base safetensors file.
         max_shard_size: Maximum size of each shard file. If the model exceeds this size,
             it will be sharded into multiple files and a .safetensors.index.json will be
             created. Use smaller values like "5GB" or "2GB" to force sharding.
@@ -879,7 +970,8 @@ def _export_diffusers_checkpoint(
 
             # Step 5: Build quantization config
             quant_config = get_quant_config(component, is_modelopt_qlora=False)
-
+            hf_quant_config = convert_hf_quant_config_format(quant_config) if quant_config else None
+            
             # Step 6: Save the component
             # - diffusers ModelMixin.save_pretrained does NOT accept state_dict parameter
             # - for non-diffusers modules (e.g., LTX-2 transformer), fall back to torch.save
@@ -888,12 +980,14 @@ def _export_diffusers_checkpoint(
                     component.save_pretrained(component_export_dir, max_shard_size=max_shard_size)
             else:
                 with hide_quantizers_from_state_dict(component):
-                    _save_component_state_dict_safetensors(component, component_export_dir)
-
+                    _save_component_state_dict_safetensors(
+                        component, 
+                        component_export_dir, 
+                        merged_base_safetensor_path,
+                        hf_quant_config,
+                    )
             # Step 7: Update config.json with quantization info
-            if quant_config is not None:
-                hf_quant_config = convert_hf_quant_config_format(quant_config)
-
+            if hf_quant_config is not None:
                 config_path = component_export_dir / "config.json"
                 if config_path.exists():
                     with open(config_path) as file:
@@ -905,7 +999,7 @@ def _export_diffusers_checkpoint(
         elif hasattr(component, "save_pretrained"):
             component.save_pretrained(component_export_dir, max_shard_size=max_shard_size)
         else:
-            _save_component_state_dict_safetensors(component, component_export_dir)
+            _save_component_state_dict_safetensors(component, component_export_dir, merged_base_safetensor_path)
 
         print(f"  Saved to: {component_export_dir}")
 
@@ -985,6 +1079,7 @@ def export_hf_checkpoint(
     save_modelopt_state: bool = False,
     components: list[str] | None = None,
     extra_state_dict: dict[str, torch.Tensor] | None = None,
+    merged_base_safetensor_path: str | None = None,
 ):
     """Export quantized HuggingFace model checkpoint (transformers or diffusers).
 
@@ -1002,6 +1097,9 @@ def export_hf_checkpoint(
         components: Only used for diffusers pipelines. Optional list of component names
             to export. If None, all quantized components are exported.
         extra_state_dict: Extra state dictionary to add to the exported model.
+        merged_base_safetensor_path: If provided, merge the exported diffusion transformer
+            with non-transformer components (VAE, vocoder, etc.) from this base safetensors
+            file. Only used for diffusion model exports (e.g., LTX-2).
     """
     export_dir = Path(export_dir)
     export_dir.mkdir(parents=True, exist_ok=True)
@@ -1010,7 +1108,7 @@ def export_hf_checkpoint(
     if HAS_DIFFUSERS:
         is_diffusers_obj = is_diffusers_object(model)
     if is_diffusers_obj:
-        _export_diffusers_checkpoint(model, dtype, export_dir, components)
+        _export_diffusers_checkpoint(model, dtype, export_dir, components, merged_base_safetensor_path)
         return
 
     # Transformers model export