message: [XLA] Move a CHECK into Layout verification
  Sparse layouts that specify tiling should fail verification, not CHECK-fail in ToString.
diff: @@ -247,6 +247,10 @@ Layout CreateDefaultLayoutForRank(int64 rank) {
  }
  dimensions_in_layout[dim] = true;
  }
  + } else {
  + if (layout.tiles_size() != 0) {
  + return InvalidArgument("Only dense layouts can be tiled.");
  + }
  }
  return Status::OK();
  