Returns path to most recently modified checkpoint.

def _find_newest_ckpt(ckpt_dir):
    """Returns path to most recently modified checkpoint."""
    full_paths = [
        os.path.join(ckpt_dir, fname) for fname in os.listdir(ckpt_dir)
        if fname.startswith("experiment_state") and fname.endswith(".json")
    ]
    return max(full_paths)