merge.py

"""Merge LoRA adapter into base model weights.

Usage:
    pip install torch transformers safetensors tqdm huggingface-hub
    python merge.py --output ./merged_model

Loads the MXFP4 base model, dequantizes to bf16, applies LoRA deltas, saves merged model.
"""

import argparse
import json
import shutil
from pathlib import Path

import torch
from huggingface_hub import snapshot_download
from safetensors.torch import load_file, save_file
from tqdm import tqdm
from transformers import AutoModelForCausalLM

BASE_MODEL = "openai/gpt-oss-120b"
ADAPTER_REPO = "LightningRodLabs/Golf-Forecaster"


def merge(output_dir: str):
    output_dir = Path(output_dir)
    output_dir.mkdir(parents=True, exist_ok=True)

    # Download adapter
    print("Downloading adapter...")
    adapter_dir = Path(snapshot_download(ADAPTER_REPO))
    adapter_config = json.loads((adapter_dir / "adapter_config.json").read_text())
    scaling = adapter_config["lora_alpha"] / adapter_config["r"]
    adapter_weights = load_file(str(adapter_dir / "adapter_model.safetensors"))
    print(f"Adapter: {len(adapter_weights)} keys, scaling={scaling}")

    # Load base model (dequantizes MXFP4 to bf16)
    print("Loading base model...")
    base_model = AutoModelForCausalLM.from_pretrained(
        BASE_MODEL, torch_dtype=torch.bfloat16, device_map="cpu", trust_remote_code=True,
    )
    state_dict = base_model.state_dict()
    del base_model

    # Group LoRA A/B pairs
    lora_pairs = {}
    for key, tensor in adapter_weights.items():
        clean = key.replace("base_model.model.", "", 1)
        if ".lora_A.weight" in clean:
            lora_pairs.setdefault(clean.replace(".lora_A.weight", ""), {})["A"] = tensor
        elif ".lora_B.weight" in clean:
            lora_pairs.setdefault(clean.replace(".lora_B.weight", ""), {})["B"] = tensor

    # Map adapter keys to base model keys + merge operation
    # Adapter uses Tinker naming; HF transformers uses different names:
    #   attn -> self_attn, w1(gate)/w3(up) -> gate_up_proj (interleaved), w2 -> down_proj
    base_key_ops = {}
    for adapter_path in lora_pairs:
        if "unembed_tokens" in adapter_path:
            base_key_ops.setdefault("lm_head.weight", []).append(("add", adapter_path))
        elif ".attn." in adapter_path:
            base_key = adapter_path.replace(".attn.", ".self_attn.") + ".weight"
            base_key_ops.setdefault(base_key, []).append(("add", adapter_path))
        elif ".mlp.experts.w1" in adapter_path:
            prefix = adapter_path.split(".mlp.experts.w1")[0]
            base_key_ops.setdefault(prefix + ".mlp.experts.gate_up_proj", []).append(("even_t", adapter_path))
        elif ".mlp.experts.w3" in adapter_path:
            prefix = adapter_path.split(".mlp.experts.w3")[0]
            base_key_ops.setdefault(prefix + ".mlp.experts.gate_up_proj", []).append(("odd_t", adapter_path))
        elif ".mlp.experts.w2" in adapter_path:
            prefix = adapter_path.split(".mlp.experts.w2")[0]
            base_key_ops.setdefault(prefix + ".mlp.experts.down_proj", []).append(("add_t", adapter_path))

    # Apply LoRA deltas
    for base_key, ops in tqdm(sorted(base_key_ops.items()), desc="Merging LoRA"):
        w = state_dict[base_key].float()
        for op_type, adapter_path in ops:
            A = lora_pairs[adapter_path]["A"].float()
            B = lora_pairs[adapter_path]["B"].float()
            delta = torch.matmul(B, A) * scaling
            if op_type == "add":
                w += delta
            elif op_type == "even_t":
                w[:, :, ::2] += delta.transpose(1, 2)
            elif op_type == "odd_t":
                w[:, :, 1::2] += delta.transpose(1, 2)
            elif op_type == "add_t":
                w += delta.transpose(1, 2)
        state_dict[base_key] = w.to(torch.bfloat16)

    # Save sharded safetensors
    print(f"Saving to {output_dir}...")
    max_shard = 5 * 1024**3
    shards, current, size = [], {}, 0
    for k, v in state_dict.items():
        nbytes = v.numel() * v.element_size()
        if size + nbytes > max_shard and current:
            shards.append(current)
            current, size = {}, 0
        current[k] = v
        size += nbytes
    if current:
        shards.append(current)

    weight_map, total = {}, 0
    for i, shard in enumerate(shards):
        fname = f"model-{i+1:05d}-of-{len(shards):05d}.safetensors"
        save_file(shard, str(output_dir / fname))
        for k, v in shard.items():
            weight_map[k] = fname
            total += v.numel() * v.element_size()

    (output_dir / "model.safetensors.index.json").write_text(
        json.dumps({"metadata": {"total_size": total}, "weight_map": weight_map}, indent=2)
    )

    # Copy config + tokenizer from base model (remove quantization_config)
    base_cache = Path(snapshot_download(BASE_MODEL, allow_patterns=["*.py", "*.json", "tokenizer*", "*.model"]))
    for f in base_cache.iterdir():
        if f.is_file() and f.name != "model.safetensors.index.json":
            shutil.copy2(f, output_dir / f.name)
    cfg = json.loads((output_dir / "config.json").read_text())
    cfg.pop("quantization_config", None)
    cfg["torch_dtype"] = "bfloat16"
    (output_dir / "config.json").write_text(json.dumps(cfg, indent=2))

    print(f"Done! Merged model saved to {output_dir} ({len(shards)} shards)")


if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument("--output", required=True, help="Output directory for merged model")
    merge(parser.parse_args().output)