Add some code to bypass multiprocessing when using a spawn context

[lopez86]

Summary: Other things I've tried include

• attaching the LM to the instance instead of the class - works but is slow for both fork and spawn since each process rebuilds the LM
• adding the language model as an optional parameter if a spawn context is found and rebuilding the model container - also works, but again is slow since each process needs its own copy As far as I can tell this is often going to be faster than the multiprocessing options that I've tried and also allows for just not using a pool at all (new feature for these functions)

This PR also does a bit of refactoring and improving some type hints.