code: protected function renderHeaderCellContent()
      {
          if ($this->header !== null || !$this->multiple) {
              return parent::renderHeaderCellContent();
          }
  
          return Html::checkbox($this->getHeaderCheckBoxName(), false, ['class' => 'select-on-check-all']);
      }
docs: Renders the header cell content.
  The default implementation simply renders [[header]].
  This method may be overridden to customize the rendering of the header cell.
  @return string the rendering result