message: Add comment re lambda in SparseFillEmptyRowsOpImpl
  This refers to compilation/linker errors that were observed in the
  github CI.
diff: @@ -193,6 +193,8 @@ namespace {
  template <typename Device, typename T, typename Tindex>
  void SparseFillEmptyRowsOpImpl(
  OpKernelContext* context, AsyncOpKernel::DoneCallback done = nullptr) {
  + // Note that setting this empty lambda as the default parameter value directly
  + // can cause strange compiler/linker errors, so we do it like this instead.
  if (!done) {
  done = [] {};
  }
  