Support pre-computing statistics for each system

[zdou0830]

No description provided.

[neubig]

Thanks for this! As I was looking through the code, it seemed that this adds a bit of repetitiveness and complexity to the codebase. I wonder if there is a way that we can create a more general wrapper for cached functions that would make this cleaner? If you can't think of a way to do it immediately then this is probably OK as-is, but I think it'd be a nice thing to have. Maybe something like the standard Python lru_cache functionality (although I don't think we could use this as-is).

[zdou0830]

Thanks! I will take a look at lru_cache and think more about it!

[zdou0830]

I just made the code a little bit cleaner, but I haven't come up with a way to create a more general wrapper yet. Please take a look. Thanks!

[neubig]

Thanks! Looks quite a bit better, I'll merge.