message: Log the start of filling up the shuffle buffer
  When debugging performance issues we only saw 2 logs, the intermediate
  state and the end. However, we did not know when the shuffle buffer
  processing started. The updated logging should be clearer and will
  record both the start and end.
diff: @@ -129,6 +129,10 @@ class ShuffleDatasetOpBase : public UnaryDatasetOpKernel {
  ctx, this->prefix(), &input_impl_));
  }
  if (!end_of_input_sequence) {
  + if (num_elements_ == 0) {
  + VLOG(1) << "Starting to fill up shuffle buffer of size: "
  + << this->dataset()->buffer_size_;
  + }
  this->RecordBufferEnqueue(ctx, input_element);
  buffer_[slices_.back()->end % this->dataset()->buffer_size_] =
  std::move(input_element);
  