Trying to print a form with a shortcode
Basically, drupal_get_form doesn't render the array - it just returns the renderable array from the form function. You need to render it yourself.

Try this:

function custom_shortcodes_shortcode_form($attrs, $text) {
  $form = drupal_get_form('custom_shortcodes_shortcode_form_attributes');
  $attrs = drupal_render($form);
  return $attrs;
}