Quick questions about details
## ❓ Questions & Help

<!-- A clear and concise description of the question. -->

Can someone explain the difference between `mem_len`,  `mlen`, and `ext_len` in `TransfoXLModel`? While the documentation has stated below

![Screen Shot 2019-09-10 at 11 18 40](https://user-images.githubusercontent.com/23093968/64583914-110f3600-d3bd-11e9-9571-5e9c607a1744.png)

Unfortunately, I'm still confused especially `mem_len` and `mlen`. Thank you so much.
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
