Add the option for the macro and note (Dao-AILab#893)

Author: drisspg

csrc/flash_attn/src/softmax.h

@@ -78,7 +78,14 @@ __forceinline__ __device__ void scale_apply_exp2(Tensor<Engine0, Layout0> &tenso
             // Instead of computing exp(x - max), we compute exp2(x * log_2(e) -
             // max * log_2(e)) This allows the compiler to use the ffma
             // instruction instead of fadd and fmul separately.
-            tensor(mi, ni) = exp2f(tensor(mi, ni) * scale - max_scaled);
+            // The following macro will disable the use of fma.
+            // See: https://github.com/pytorch/pytorch/issues/121558 for more details
+            // This macro is set in PyTorch and not FlashAttention
+            #ifdef UNFUSE_FMA
+                tensor(mi, ni) = exp2f(__fmul_rn(tensor(mi, ni), scale) - max_scaled);
+            #else
+                tensor(mi, ni) = exp2f(tensor(mi, ni) * scale - max_scaled);
+            #endif
         }
     }
 }