Add kIsSingleSource to PairwiseDistanceEpilogueElementwise

With raft having recently migrated to cutlass 3.5.1, this field is
now required.

cpp/src/distance/detail/pairwise_distance_epilogue_elementwise.h

@@ -61,6 +61,7 @@ class PairwiseDistanceEpilogueElementwise {
   using ElementT                      = ElementT_;
   static int const kElementsPerAccess = ElementsPerAccess;
   static int const kCount             = kElementsPerAccess;
+  static bool const kIsSingleSource   = true;
 
   using DistanceOp = DistanceOp_;
   using FinalOp    = FinalOp_;