[Feature] Support Flashinfer fmha on Blackwell (#6930)

python/sglang/srt/layers/attention/flashinfer_backend.py

@@ -25,6 +25,7 @@ from sglang.global_config import global_config
 from sglang.srt.layers.attention.base_attn_backend import AttentionBackend
 from sglang.srt.layers.attention.utils import create_flashinfer_kv_indices_triton
 from sglang.srt.layers.dp_attention import get_attention_tp_size
+from sglang.srt.layers.utils import is_sm100_supported
 from sglang.srt.model_executor.forward_batch_info import ForwardBatch, ForwardMode
 from sglang.srt.speculative.eagle_utils import EagleDraftInput, EagleVerifyInput
 from sglang.srt.utils import is_flashinfer_available, next_power_of_2
@@ -149,8 +150,11 @@ class FlashInferAttnBackend(AttentionBackend):
                 for _ in range(self.num_wrappers)
             ]
 
+        fmha_backend = "auto"
+        if is_sm100_supported():
+            fmha_backend = "cutlass"
         self.prefill_wrapper_ragged = BatchPrefillWithRaggedKVCacheWrapper(
-            self.workspace_buffer, "NHD"
+            self.workspace_buffer, "NHD", backend=fmha_backend
         )
 
         # Two wrappers: one for sliding window attention and one for full attention.