fix: correct fused KV cache stride in paged MQA logits

[JasonOA888]

Bug

fp8_fp4_paged_mqa_logits decomposes the fused KV cache into separate kv_cache and kv_cache_sf tensors via torch::from_blob. The strides passed to from_blob assume a flat layout (all values then all SF), but the API assertion enforces an interleaved layout ([values | SF] per row):

DG_HOST_ASSERT(fused_kv_cache.stride(1) == fp4_with_sf_bytes);  // FP4: 68
DG_HOST_ASSERT(fused_kv_cache.stride(1) == head_dim_with_sf);  // FP8: 132

What was wrong

kv_cache stride(1):

• FP4: was head_dim/2 (64), should be fp4_with_sf_bytes (68)
• FP8: was head_dim (128), should be head_dim_with_sf (132)

kv_cache_sf offset:

• Was block_kv * head_dim, pointing past all data rows into undefined territory
• Should be head_dim (or head_dim/2 for FP4), pointing to the SF of the first row

kv_cache_sf stride(1):

• Was 1 (contiguous), but SF values are interleaved every fp4_with_sf_bytes/head_dim_with_sf bytes

Impact

TMA descriptors constructed with wrong strides cause misaligned reads for any row > 0 in the paged KV cache. This silently corrupts attention logits for multi-row blocks.

The test helper had the same flat-layout assumption, masking the bug for small block sizes where corruption fell in the last 1-2 rows.

Fix

• Use the correct row stride (fp4_with_sf_bytes / head_dim_with_sf) for both kv_cache and kv_cache_sf from_blob
• Fix kv_cache_sf offset to head_dim / head_dim / 2 (start of first row's SF)
• Fix test helpers to construct proper interleaved layout