XCiT: Cross-Covariance Image Transformers - Ali, Alaaeldin; Touvron, Hugo; Caron, Mathilde; Bojanowski, Piotr; Douze, Matthijs; Joulin, Armand; Laptev, Ivan; Neverova, Natalia; Synnaeve, Gabriel; Verbeek, Jakob; Jegou, Herve (Advances in Neural Information Processing Systems, 2021)