model_loader.py

# model_loader.py
import os
from pathlib import Path
import torch

def load_model_and_tokenizer(model_repo_dir_or_local_path: str, base_model_id: str = "google/muril-base-cased", device: str = None):
    """
    Robust loader that:
      - loads full model if model.safetensors or pytorch_model.bin exists in model_repo_dir_or_local_path
      - otherwise loads base_model_id then applies PEFT adapter from adapter_model.safetensors (if present)
    Returns (tokenizer, model_on_device, backend_str)
    """
    device = device or ("cuda" if torch.cuda.is_available() else "cpu")
    model_dir = Path(model_repo_dir_or_local_path)

    # prefer safetensors full model if present
    full_model_files = ["model.safetensors", "pytorch_model.bin"]
    adapter_files = ["adapter_model.safetensors", "adapter_config.json", "adapter.safetensors"]

    # Import delayed to allow environment to control package errors
    from transformers import AutoTokenizer, AutoModel, AutoModelForSequenceClassification

    tokenizer = None
    model = None

    # 1) Try to load full model from model_dir
    for f in full_model_files:
        full_path = model_dir / f
        if full_path.exists():
            tokenizer = AutoTokenizer.from_pretrained(str(model_dir), use_fast=True)
            try:
                model = AutoModel.from_pretrained(str(model_dir), trust_remote_code=False)
                backend = "full-AutoModel"
            except Exception:
                model = AutoModelForSequenceClassification.from_pretrained(str(model_dir), trust_remote_code=False)
                backend = "full-AutoModelForSequenceClassification"
            model.to(device)
            model.eval()
            return tokenizer, model, backend

    # 2) If full model not present, check for adapter files
    has_adapter = any((model_dir / af).exists() for af in adapter_files)
    if has_adapter:
        try:
            tokenizer = AutoTokenizer.from_pretrained(str(model_dir), use_fast=True)
        except Exception:
            tokenizer = AutoTokenizer.from_pretrained(base_model_id, use_fast=True)

        try:
            base = AutoModel.from_pretrained(str(model_dir))
            base_name = str(model_dir)
        except Exception:
            base = AutoModel.from_pretrained(base_model_id)
            base_name = base_model_id

        base.to(device)
        base.eval()

        try:
            from peft import PeftModel
            adapter_dir = str(model_dir)
            # If adapter files exist, PeftModel.from_pretrained will pick them up
            peft_model = PeftModel.from_pretrained(base, adapter_dir, is_trainable=False)
            peft_model.to(device)
            peft_model.eval()
            return tokenizer, peft_model, "peft-attached"
        except Exception as e:
            raise RuntimeError(f"Failed to load/apply PEFT adapter from {model_dir}: {e}") from e

    # 3) Fallback: try direct load (may fail)
    try:
        tokenizer = AutoTokenizer.from_pretrained(str(model_dir), use_fast=True)
        model = AutoModel.from_pretrained(str(model_dir))
        model.to(device)
        model.eval()
        return tokenizer, model, "auto-fallback"
    except Exception as e:
        raise RuntimeError(f"Unable to load model or adapters from {model_dir}. Error: {e}") from e