code: protected function setFormClass(FormInterface $oForm, $sFormLayout = self::LAYOUT_HORIZONTAL)
      {
          if (is_string($sFormLayout)) {
              $sLayoutClass = 'form-' . $sFormLayout;
              if ($sFormClass = $oForm->getAttribute('class')) {
                  if (!preg_match('/(\s|^)' . preg_quote($sLayoutClass, '/') . '(\s|$)/', $sFormClass)) {
                      $oForm->setAttribute('class', trim($sFormClass . ' ' . $sLayoutClass));
                  }
              } else {
                  $oForm->setAttribute('class', $sLayoutClass);
              }
          }
          return $this;
      }
docs: Sets form layout class
  @param FormInterface $oForm
  @param string $sFormLayout
  @return \TwbBundle\Form\View\Helper\TwbBundleForm