[Perf] Set Flashinfer sparse MLA as default backend for FP8 kv cache (#37252)

Signed-off-by: wzhao18 <wzhao18.sz@gmail.com>

tools/pre_commit/generate_attention_backend_docs.py

@@ -1262,14 +1262,23 @@ When no backend is specified (the default):
 """
 
 
-def _priority_table(title: str, backends: list[str]) -> list[str]:
+def _priority_table(
+    title: str,
+    backends: list[str],
+    annotations: dict[str, str] | None = None,
+) -> list[str]:
     """Generate a priority table for a list of backends."""
+
+    def _fmt(b: str) -> str:
+        suffix = annotations.get(b, "") if annotations else ""
+        return f"`{b}`{suffix}"
+
     return [
         f"**{title}:**",
         "",
         "| Priority | Backend |",
         "| -------- | ------- |",
-        *[f"| {i} | `{b}` |" for i, b in enumerate(backends, 1)],
+        *[f"| {i} | {_fmt(b)} |" for i, b in enumerate(backends, 1)],
         "",
     ]
 
@@ -1298,11 +1307,25 @@ def generate_priority_section(priorities: dict[str, list[str]]) -> str:
 
     lines.extend(["### MLA Attention (DeepSeek-style)", ""])
 
+    mla_sm100_annotations = {
+        "FLASHINFER_MLA_SPARSE": "**\\***",
+    }
     if "mla_sm100" in priorities:
-        lines.extend(_priority_table(sm100, priorities["mla_sm100"]))
+        lines.extend(
+            _priority_table(sm100, priorities["mla_sm100"], mla_sm100_annotations)
+        )
     if "mla_default" in priorities:
         lines.extend(_priority_table(ampere, priorities["mla_default"]))
 
+    if "mla_sm100" in priorities:
+        lines.append(
+            "> **\\*** For sparse MLA, FP8 KV cache always prefers "
+            "`FLASHINFER_MLA_SPARSE`. With BF16 KV cache, `FLASHINFER_MLA_SPARSE` "
+            "is preferred for low query-head counts (<= 16), while "
+            "`FLASHMLA_SPARSE` is preferred otherwise."
+        )
+        lines.append(">")
+
     lines.append(
         "> **Note:** ROCm and CPU platforms have their own selection logic. "
         "See the platform-specific documentation for details."