style: fix ruff lint warnings

- Remove unused local variable b in attention_backend.py
- Remove unused variable rank0_sd in test_broadcast_state_dict.py
- Remove unused imports across test files

tests/data/test_dataset.py

@@ -6,7 +6,6 @@ import numpy as np
 import pytest
 import torch
 
-from astrai.config.preprocess_config import PipelineConfig
 from astrai.dataset.dataset import (
     DatasetFactory,
     GRPODataset,