[chore] add a script to extract loras from full fine-tuned models (#10631)

* feat: add a lora extraction script.

* updates

scripts/extract_lora_from_model.py

+"""
+This script demonstrates how to extract a LoRA checkpoint from a fully finetuned model with the CogVideoX model.
+
+To make it work for other models:
+
+* Change the model class. Here we use `CogVideoXTransformer3DModel`. For Flux, it would be `FluxTransformer2DModel`,
+for example. (TODO: more reason to add `AutoModel`).
+* Spply path to the base checkpoint via `base_ckpt_path`.
+* Supply path to the fully fine-tuned checkpoint via `--finetune_ckpt_path`.
+* Change the `--rank` as needed.
+
+Example usage:
+
+```bash
+python extract_lora_from_model.py \
+    --base_ckpt_path=THUDM/CogVideoX-5b \
+    --finetune_ckpt_path=finetrainers/cakeify-v0 \
+    --lora_out_path=cakeify_lora.safetensors
+```
+
+Script is adapted from
+https://github.com/Stability-AI/stability-ComfyUI-nodes/blob/001154622564b17223ce0191803c5fff7b87146c/control_lora_create.py
+"""
+
+import argparse
+
+import torch
+from safetensors.torch import save_file
+from tqdm.auto import tqdm
+
+from diffusers import CogVideoXTransformer3DModel
+
+
+RANK = 64
+CLAMP_QUANTILE = 0.99
+
+
+# Comes from
+# https://github.com/Stability-AI/stability-ComfyUI-nodes/blob/001154622564b17223ce0191803c5fff7b87146c/control_lora_create.py#L9
+def extract_lora(diff, rank):
+    # Important to use CUDA otherwise, very slow!
+    if torch.cuda.is_available():
+        diff = diff.to("cuda")
+
+    is_conv2d = len(diff.shape) == 4
+    kernel_size = None if not is_conv2d else diff.size()[2:4]
+    is_conv2d_3x3 = is_conv2d and kernel_size != (1, 1)
+    out_dim, in_dim = diff.size()[0:2]
+    rank = min(rank, in_dim, out_dim)
+
+    if is_conv2d:
+        if is_conv2d_3x3:
+            diff = diff.flatten(start_dim=1)
+        else:
+            diff = diff.squeeze()
+
+    U, S, Vh = torch.linalg.svd(diff.float())
+    U = U[:, :rank]
+    S = S[:rank]
+    U = U @ torch.diag(S)
+    Vh = Vh[:rank, :]
+
+    dist = torch.cat([U.flatten(), Vh.flatten()])
+    hi_val = torch.quantile(dist, CLAMP_QUANTILE)
+    low_val = -hi_val
+
+    U = U.clamp(low_val, hi_val)
+    Vh = Vh.clamp(low_val, hi_val)
+    if is_conv2d:
+        U = U.reshape(out_dim, rank, 1, 1)
+        Vh = Vh.reshape(rank, in_dim, kernel_size[0], kernel_size[1])
+    return (U.cpu(), Vh.cpu())
+
+
+def parse_args():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "--base_ckpt_path",
+        default=None,
+        type=str,
+        required=True,
+        help="Base checkpoint path from which the model was finetuned. Can be a model ID on the Hub.",
+    )
+    parser.add_argument(
+        "--base_subfolder",
+        default="transformer",
+        type=str,
+        help="subfolder to load the base checkpoint from if any.",
+    )
+    parser.add_argument(
+        "--finetune_ckpt_path",
+        default=None,
+        type=str,
+        required=True,
+        help="Fully fine-tuned checkpoint path. Can be a model ID on the Hub.",
+    )
+    parser.add_argument(
+        "--finetune_subfolder",
+        default=None,
+        type=str,
+        help="subfolder to load the fulle finetuned checkpoint from if any.",
+    )
+    parser.add_argument("--rank", default=64, type=int)
+    parser.add_argument("--lora_out_path", default=None, type=str, required=True)
+    args = parser.parse_args()
+
+    if not args.lora_out_path.endswith(".safetensors"):
+        raise ValueError("`lora_out_path` must end with `.safetensors`.")
+
+    return args
+
+
+@torch.no_grad()
+def main(args):
+    model_finetuned = CogVideoXTransformer3DModel.from_pretrained(
+        args.finetune_ckpt_path, subfolder=args.finetune_subfolder, torch_dtype=torch.bfloat16
+    )
+    state_dict_ft = model_finetuned.state_dict()
+
+    # Change the `subfolder` as needed.
+    base_model = CogVideoXTransformer3DModel.from_pretrained(
+        args.base_ckpt_path, subfolder=args.base_subfolder, torch_dtype=torch.bfloat16
+    )
+    state_dict = base_model.state_dict()
+    output_dict = {}
+
+    for k in tqdm(state_dict, desc="Extracting LoRA..."):
+        original_param = state_dict[k]
+        finetuned_param = state_dict_ft[k]
+        if len(original_param.shape) >= 2:
+            diff = finetuned_param.float() - original_param.float()
+            out = extract_lora(diff, RANK)
+            name = k
+
+            if name.endswith(".weight"):
+                name = name[: -len(".weight")]
+            down_key = "{}.lora_A.weight".format(name)
+            up_key = "{}.lora_B.weight".format(name)
+
+            output_dict[up_key] = out[0].contiguous().to(finetuned_param.dtype)
+            output_dict[down_key] = out[1].contiguous().to(finetuned_param.dtype)
+
+    prefix = "transformer" if "transformer" in base_model.__class__.__name__.lower() else "unet"
+    output_dict = {f"{prefix}.{k}": v for k, v in output_dict.items()}
+    save_file(output_dict, args.lora_out_path)
+    print(f"LoRA saved and it contains {len(output_dict)} keys.")
+
+
+if __name__ == "__main__":
+    args = parse_args()
+    main(args)