[Feature] Support FA3 backend for MLA (#4831)

python/sglang/srt/models/deepseek_v2.py

@@ -655,6 +655,7 @@ class DeepseekV2AttentionMLA(nn.Module):
         self.flashinfer_mla_disable_ragged = global_server_args_dict[
             "flashinfer_mla_disable_ragged"
         ]
+        self.attention_backend = global_server_args_dict["attention_backend"]
         self.rocm_fused_decode_mla = os.getenv("SGLANG_ROCM_FUSED_DECODE_MLA") == "1"
 
     def no_absorb(self, forward_batch: ForwardBatch) -> bool:
@@ -667,6 +668,9 @@ class DeepseekV2AttentionMLA(nn.Module):
                 and not forward_batch.forward_mode.is_draft_extend()
                 and sum(forward_batch.extend_prefix_lens_cpu) == 0
             )
+        elif self.attention_backend == "fa3":
+            # Flash Attention: Keep absorbing for all extend/decode
+            return False
         else:
             # Triton: Use normal computation for prefill and use weight absorption for extend/decode
             return (