Can I get answer for my confusion in week07_seq2seq/practice_torch

I am confused deeply about logp_sample and entropy in Function scst_objective_on_batch in week07_seq2seq/practice_torch. But there is no correct answer as a reference. Recently I want to reproduce the seq2seq with RL like week07 task, but I'm stuck in these two places. Many thanks to the all teachers of the course group. I hope someone can answer my confusion.