bpe-openai: allow lazy loading BPE and feature flag each model

[howardjohn]

The fully generated .dict models take 70mb, which bloats binaries substantially. This is compared to 3.4mb for the targz.

It would be nice to support lazily doing this translation, which takes a modest hit to runtime performance in return for dramatically smaller binary, and the ability to only load certain models reducing memory consumption.

It would also be nice to have feature flags to turn off certain models; this is less important if we add lazy loading, though, as it would just allow you to save 1-2mb off the binary.

[gorzell]

Hi @howardjohn, generally speaking pull requests are welcome. I wonder if both variations could actually be served/represented by feature flags or if that would make things too complicated. Our use case isn't super sensitive to binary size (obviously 😉) but making this more configurable seems reasonable. We also have to figure out if we want to keep using the openai crate as a catch all for every variant.

[howardjohn]

Sounds good! I'll look into this in the coming weeks and see if I can put together a PR. Thanks for the great library