perf: accelerate decode linear with bf16 gemv

- add decode-shape benchmark harness
- add bf16 GEMV CUDA primitive with head-dim generic kernel
- dispatch decode-time linear layers to gemv for M=1
- extend gemv coverage to small decode batches

astrai/extension/backend/__init__.py

@@ -11,6 +11,7 @@ from astrai.extension.backend.attention import (
     attn_backend,
     get_backend,
 )
+from astrai.extension.backend.linear import linear
 from astrai.extension.backend.rotary import apply_rotary_emb
 
 __all__ = [
@@ -24,4 +25,5 @@ __all__ = [
     "attention",
     "attn_backend",
     "get_backend",
+    "linear",
 ]