Efficient Content-Based Sparse Attention with Routing Transformers Aurko Roy author Mohammad Saffar author Ashish Vaswani author David Grangier author 2021 text journal article Transactions of the Association for Computational Linguistics continuing MIT Press Cambridge, MA periodical academic journal roy-etal-2021-efficient 10.1162/tacl_a_00353 https://aclanthology.org/2021.tacl-1.4/ 2021 9 53 68