困惑度 PPL perplexity

https://huggingface.co/docs/transformers/perplexity

https://huggingface.co/spaces/evaluate-metric/perplexity

https://towardsdatascience.com/perplexity-in-language-models-87a196019a94

https://cloud.tencent.com/developer/article/1602293

https://stats.stackexchange.com/questions/402686/word-perplexity-on-a-subword-language-model

https://stats.stackexchange.com/questions/247842/calculating-test-time-perplexity-for-seq2seq-rnn-language-models

https://docs.chainer.org/en/stable/examples/ptb.html

https://www.jianshu.com/p/9abeb65d1d10

https://zhuanlan.zhihu.com/p/273606445

http://fancyerii.github.io/dev287x/lm/#%E6%A8%A1%E5%9E%8B%E8%AF%84%E4%BC%B0

ngram