perf: remove split partials memset and overlap decode tile loads

- alloc_split_partials now uses torch::empty: the split kernel writes every slot it owns, so the per-call zeros/full memset was pure overhead (2 kernels per layer per step)
- decode split-KV MMA kernels now run a true multi-stage cp.async pipeline (wait_group<STAGES-1> instead of wait_group<0>), keeping STAGES-1 tile loads in flight; the old wait_group<0> serialized load and compute so deeper STAGES made no difference
- add a fallback path when ntiles < STAGES to avoid a race on the last tile

astrai/extension/attention_backend.py

@@ -30,7 +30,6 @@ Layout convention: all q/k/v are ``[batch, seq_len, n_heads, head_dim]``
 
 import contextvars
 import enum
-import math
 from abc import ABC, abstractmethod
 from contextlib import contextmanager
 from typing import Optional, Union