Couple of things wrong here:
// view:
@foreach($qrs as $qr->rating)
// should be:
@foreach($qrs as $qr)
// controller:
$unit = Unit::all()->first();
$qrs = Qr::all()->first();
// this way you get all Units then fetch first Unit from the collection,
// the same with Qrs, so change it to:
$unit = Unit::all(); // do you need it at all?
$qrs = Qr::with('ratings')->get();
This will solve the problem and in the foreach loop you will be able to access $qr->ratings->count() which will be Collection method.