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/ops/__init__.py

@@ -7,6 +7,7 @@ from astrai.extension.ops.attention import (
     attn_paged_prefill,
     attn_prefill,
 )
+from astrai.extension.ops.gemv import bf16_gemv
 from astrai.extension.ops.rotary import rotary_emb
 
 __all__ = [
@@ -15,5 +16,6 @@ __all__ = [
     "attn_paged_decode",
     "attn_paged_prefill",
     "attn_prefill",
+    "bf16_gemv",
     "rotary_emb",
 ]