Differentiable Subset Pruning of Transformer Heads Jiaoda Li author Ryan Cotterell author Mrinmaya Sachan author 2021 text journal article Transactions of the Association for Computational Linguistics continuing MIT Press Cambridge, MA periodical academic journal li-etal-2021-differentiable 10.1162/tacl_a_00436 https://aclanthology.org/2021.tacl-1.86/ 2021 9 1442 1459