lora_redim.py

import os
import argparse
import torch
from safetensors.torch import load_file, save_file
from safetensors import safe_open
from tqdm import tqdm


def resize_lora_model(model_path, output_path, new_dim, device):
    """
    Resizes the LoRA dimension of a model using SVD for optimal weight preservation.

    Args:
        model_path (str): Path to the LoRA model to resize.
        output_path (str): Path to save the new resized model.
        new_dim (int): The target new dimension for the LoRA weights.
        device (str): The device to run calculations on ('cuda' or 'cpu').
    """
    print(f"Loading model from: {model_path}")
    model = load_file(model_path)
    new_model = {}

    # --- Metadata & Weight Inspection ---
    original_dim = None
    alpha = None
    try:
        with safe_open(model_path, framework="pt", device="cpu") as f:
            metadata = f.metadata()
            if metadata:
                if 'ss_network_dim' in metadata:
                    original_dim = int(metadata['ss_network_dim'])
                    print(f"Original dimension (from metadata): {original_dim}")
                if 'ss_network_alpha' in metadata:
                    alpha = float(metadata['ss_network_alpha'])
                    print(f"Original alpha (from metadata): {alpha}")
    except Exception as e:
        print(f"Could not read metadata: {e}. Dimension and alpha will be inferred.")

    # Infer original_dim from weights if not in metadata
    if original_dim is None:
        for key in model.keys():
            if key.endswith((".lora_down.weight", ".lora_A.weight")):
                original_dim = model[key].shape[0]
                print(f"Inferred original dimension from weights: {original_dim}")
                break

    # Infer alpha from weights if not in metadata
    if alpha is None:
        for key in model.keys():
            if key.endswith(".alpha"):
                alpha = model[key].item()
                print(f"Inferred alpha from weights: {alpha}")
                break

                # Fallback for alpha if still not found
    if alpha is None and original_dim is not None:
        alpha = float(original_dim)
        print(f"Alpha not found, falling back to using dimension: {alpha}")

    # --- Tensor Processing ---
    lora_keys_to_process = set()
    for key in model.keys():
        if 'lora_' in key and key.endswith('.weight'):
            # Get the base name (e.g., "lora_unet_down_blocks_0_attentions_0_proj_in")
            base_key = key.split('.lora_')[0]
            lora_keys_to_process.add(base_key)

    if not lora_keys_to_process:
        print("Error: No LoRA weights found in the model.")
        return

    print(f"\nFound {len(lora_keys_to_process)} LoRA modules to resize...")

    for base_key in tqdm(sorted(list(lora_keys_to_process)), desc="Resizing modules"):
        try:
            down_key, up_key = None, None

            # Determine naming convention
            if base_key + ".lora_down.weight" in model:
                down_key = base_key + ".lora_down.weight"
                up_key = base_key + ".lora_up.weight"
            elif base_key + ".lora_A.weight" in model:
                down_key = base_key + ".lora_A.weight"
                up_key = base_key + ".lora_B.weight"
            else:
                continue

            # Move weights to the selected device for calculation
            down_weight = model[down_key].to(device)
            up_weight = model[up_key].to(device)

            # --- SVD Resizing ---
            original_dtype = up_weight.dtype

            # Combine the two matrices to get the full weight update
            conv2d = down_weight.ndim == 4
            if conv2d:
                # For conv layers, treat spatial dims as batch dims
                down_weight = down_weight.flatten(1)
                up_weight = up_weight.flatten(1)

            full_weight = up_weight @ down_weight

            # Always cast to float32 for SVD, as some devices (CPU, and some GPUs) don't support bfloat16
            U, S, Vh = torch.linalg.svd(full_weight.to(torch.float32))

            # Truncate or pad the SVD components
            U = U[:, :new_dim]
            S = S[:new_dim]
            Vh = Vh[:new_dim, :]

            # Reconstruct the new low-rank matrices
            new_down = torch.diag(S) @ Vh
            new_up = U

            # Reshape back to original conv format if necessary
            if conv2d:
                new_down = new_down.reshape(new_dim, down_weight.shape[1], 1, 1)
                new_up = new_up.reshape(up_weight.shape[0], new_dim, 1, 1)

            # Move back to CPU and original dtype for saving
            new_model[down_key] = new_down.contiguous().to(original_dtype).cpu()
            new_model[up_key] = new_up.contiguous().to(original_dtype).cpu()

            # Copy alpha tensor if it exists for this key
            alpha_key = base_key + ".alpha"
            if alpha_key in model:
                new_model[alpha_key] = model[alpha_key]

        except KeyError:
            continue

    # Copy non-LoRA tensors
    for key, value in model.items():
        if ".lora_" not in key:
            new_model[key] = value

    # --- Save New Model ---
    new_metadata = {'ss_network_dim': str(new_dim)}
    if alpha is not None and original_dim is not None and original_dim > 0:
        new_alpha = alpha * (new_dim / original_dim)
        new_metadata['ss_network_alpha'] = str(new_alpha)
        print(f"\nNew alpha scaled to: {new_alpha:.2f}")

    print(f"\nSaving resized model to: {output_path}")
    save_file(new_model, output_path, metadata=new_metadata)
    print("Done!")


if __name__ == "__main__":
    parser = argparse.ArgumentParser(
        description="Resize a LoRA model to a new dimension using SVD.",
        formatter_class=argparse.RawTextHelpFormatter
    )
    parser.add_argument("model_path", type=str, help="Path to the LoRA model (.safetensors).")
    parser.add_argument("output_path", type=str, help="Path to save the resized LoRA model.")
    parser.add_argument("new_dim", type=int, help="The new LoRA dimension (rank).")
    parser.add_argument("--device", type=str, default=None,
                        help="Device to use (e.g., 'cpu', 'cuda'). Autodetects if not specified.")

    args = parser.parse_args()

    if args.device:
        device = args.device
    else:
        device = "cuda" if torch.cuda.is_available() else "cpu"

    print(f"Using device: {device}")

    resize_lora_model(args.model_path, args.output_path, args.new_dim, device)