https://github.com/facebook/Stack-RNN/blob/95efe9031aa473e3b0b1750f5e055aac5a2b2599/train_add.cpp#L297
https://github.com/facebook/Stack-RNN/blob/95efe9031aa473e3b0b1750f5e055aac5a2b2599/train_add.cpp#L297