[Attention] Use FA4 for MLA prefill (#34732)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

benchmarks/attention_benchmarks/configs/mla_prefill.yaml

@@ -1,4 +1,19 @@
-# MLA prefill-only benchmark configuration for sparse backends
+# MLA prefill backend comparison
+#
+# Compares all available MLA prefill backends:
+#   FA backends:  fa2, fa3, fa4 (FlashAttention versions)
+#   Non-FA:       flashinfer, cudnn, trtllm (Blackwell-only, require flashinfer)
+#
+# Uses cutlass_mla as the decode backend for impl construction
+# (only the prefill path is exercised).
+#
+# Backends that aren't available on the current platform will report errors
+# in the results table (e.g., fa3 on Blackwell, cudnn without artifactory).
+#
+# Usage:
+#   python benchmark.py --config configs/mla_prefill.yaml
+
+description: "MLA prefill backend comparison"
 
 model:
   name: "deepseek-v3"
@@ -12,20 +27,25 @@ model:
   v_head_dim: 128
   block_size: 128
 
-# Model parameter sweep: simulate tensor parallelism by varying num_q_heads
-# TP=1: 128 heads, TP=2: 64 heads, TP=4: 32 heads, TP=8: 16 heads
-model_parameter_sweep:
-  param_name: "num_q_heads"
-  values: [128, 64, 32, 16]
-  label_format: "{backend}_{value}h"
+# model:
+#   name: "deepseek-v2-lite"
+#   num_layers: 27
+#   num_q_heads: 16
+#   num_kv_heads: 1
+#   head_dim: 576
+#   kv_lora_rank: 512
+#   qk_nope_head_dim: 128
+#   qk_rope_head_dim: 64
+#   v_head_dim: 128
+#   block_size: 128
 
 batch_specs:
   # Pure prefill
-  - "1q512"
-  - "1q1k"
-  - "1q2k"
-  - "1q4k"
-  - "1q8k"
+  - "q512"
+  - "q1k"
+  - "q2k"
+  - "q4k"
+  - "q8k"
 
   # Batched pure prefill
   - "2q512"
@@ -44,19 +64,63 @@ batch_specs:
   - "8q4k"
   - "8q8k"
 
-  # Extend
-  - "1q512s4k"
-  - "1q512s8k"
-  - "1q1ks8k"
-  - "1q2ks8k"
-  - "1q2ks16k"
-  - "1q4ks16k"
+  # Chunked prefill / extend
+  # Short context
+  - "q128s1k"
+  - "q256s2k"
+  - "q512s4k"
+  - "q1ks4k"
+  - "q2ks8k"
+  - "2q128s1k"
+  - "2q256s2k"
+  - "2q512s4k"
+  - "2q1ks4k"
+  - "2q2ks8k"
+  - "4q128s1k"
+  - "4q256s2k"
+  - "4q512s4k"
+  - "4q1ks4k"
+  - "4q2ks8k"
+  - "8q128s1k"
+  - "8q256s2k"
+  - "8q512s4k"
+  - "8q1ks4k"
 
-backends:
-  - FLASHMLA_SPARSE
-  - FLASHINFER_MLA_SPARSE
+  # Medium context
+  - "q128s16k"
+  - "q512s16k"
+  - "q1ks16k"
+  - "q2ks16k"
+  - "2q128s16k"
+  - "2q512s16k"
+  - "2q1ks16k"
+  - "2q2ks16k"
+  - "4q128s16k"
+  - "4q512s16k"
+  - "4q1ks16k"
+  - "4q2ks16k"
+
+  # Long context
+  - "q128s64k"
+  - "q512s64k"
+  - "q1ks64k"
+  - "q2ks64k"
+  - "2q128s64k"
+  - "2q512s64k"
+  - "2q1ks64k"
+  - "2q2ks64k"
+
+decode_backends:
+  - CUTLASS_MLA
+
+prefill_backends:
+  - fa2
+  - fa3
+  - fa4
+  - flashinfer
+  - cudnn
+  - trtllm
 
 device: "cuda:0"
-repeats: 10
-warmup_iters: 3
-profile_memory: true
+repeats: 20
+warmup_iters: 5