import torch
import torch.nn as nn


def load_checkpoint(model: nn.Module, filename: str, device: torch.device | None = None) -> dict:
    """Load model weights from a training checkpoint."""
    device = device or torch.device("cpu")
    checkpoint = torch.load(filename, map_location=device, weights_only=False)

    if not isinstance(checkpoint, dict) or "state_dict" not in checkpoint:
        raise ValueError(f"Checkpoint at {filename} does not contain a 'state_dict' key.")

    model.load_state_dict(checkpoint["state_dict"], strict=True)
    return checkpoint