feat(kernels): batched decode attention
decode_attn was hardcoded to batch 1 (a single shared write_pos, flat
[heads, dim] query). One block now covers a (row, query-head) pair with
the grid spanning batch * heads, K/V index by row, and write_pos accepts
a shared scalar or one entry per row — so batched decode rides the
kernel instead of falling back to SDPA against the full-width static
cache (the fallback measured 54% of vLLM at b=8, 2K context). The
reference implementation and kernel test cover b=1, b=4 shared-pos, and
b=4 per-row-pos.
Signed-off-by: Guokai Ma <guokai.ma@intel.com>
