[Tracing Issue] Multi-head Latent Attention

[yzh119]

Background

We need two sets of kernels for MLA:

1. self-attention on ragged tensor, w/o matrix absorption: head_dim_qk=192, head_dim_vo=128
2. cross-attention on paged-kv cache, w/ matrix absorption: head_dim_qk=576, head_dim_vo=512 (K=V)

and serving engines are expected to use different kernels according to use cases:

1. For decoding, use 2
2. For prefilling (w/o prefix-caching), use 1
3. For incremental prefilling/chunked-prefill, use the 1+2:
   • o_1, lse_1 = cross_attention(c_q, q_pe, c_kv) (c_q: (n, 128, 512), q_pe: (n, 128, 64), c_kv: (n_kv, 576), o_1: (n, 128, 512), lse_1: (n, 128))
   • o_2, lse_2 = self_attention(q, k, v_new) (q: (n, 128, 192), k: (n, 128, 192), v: (n, 128, 128), o_2: (n, 128, 128), lse_2: (n, 128))
   • o, lse = merge(W_UV(o_1), lse_1, o_2, lse_2)

Milestone

• Prefill attention kernel for self-attention (w/o matrix absorption) feat: support deepseek prefill attention shape #765 @yzh119
• Decode attention kernel (w/ matrix absorption, page table) on CUDA Cores feat: support MLA decode #551 @tsu-bin
• General (prefill+append+decode) MLA attention kernel on FA2 template (w/ matrix absorption, page table) perf: memory efficient deepseek mla fused page-attention kernel #804 @yzh119
• General (prefill+append+decode) MLA attention kernel on FA3 template (w/ matrix absorption, page table) @yzh119 perf: FlashAttention-3 style MLA PageAttention #887