title: Different cell size in UICollectionview
body: <p>In my iphone app,I am showing images on a collection view.
  I fetch the images from urls,with urls I get the image size also.
  eg:- let's say there are two kinds of images landscape and portrait.I get value P for portrait images and L  for landscape images</p>
  
  <p>How to customize the collectionView cell's size ?</p>