Currentchoice_attr description:

Use this to add additional HTML attributes to each choice.This can be an array of attributes (if they are the same for each choice), a callable or a property path (just like choice_label).

Actually it's not possible to set attributes for all choices at once. You need to redefine attributes for each choice separately:

'choice_attr' => ['Choice A' => ['class' =>'foo'],'Choice B' => ['class' =>'foo'],    ...'Choice Z' => ['class' =>'foo'],],

Please correct me if I'm wrong.