message: [tf.data] Check if node_ is present before returning num_elemets for iterator.
diff: @@ -529,7 +529,10 @@ class IteratorBase {
  }
  // Returns the number of elements produced by this itertaor.
  - int64 num_elements() const { return node_->num_elements(); }
  + int64 num_elements() const {
  + if (node_) return node_->num_elements();
  + return 0;
  + }
  private:
  friend class DatasetBase; // for access to `AddCleanupFunction`
  