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/extension/conftest.py

@@ -5,7 +5,6 @@ import torch
 
 from astrai.config.model_config import AutoRegressiveLMConfig
 from astrai.model.transformer import AutoRegressiveLM
-from tests.conftest import skip_no_kernel
 
 D = 64
 CFG = dict(

tests/extension/test_backend.py

@@ -10,7 +10,6 @@ from astrai.extension import (
     ATTN_BACKEND,
     AttentionBackendFactory,
     CudaBackend,
-    TorchNativeBackend,
     attn_backend,
     get_backend,
 )

tests/extension/test_backend_equivalence.py

@@ -31,7 +31,6 @@ def test_training_forward_matches_torch(cuda_model):
     or non-bf16 inputs it falls back to torch SDPA.  Verify the fallback
     path matches the torch-native forward exactly.
     """
-    import pytest
 
     model, _ = cuda_model
     input_ids = torch.randint(0, 1000, (2, 16), device="cuda")