Allow --include_path to import an externally-defined LM subclass

[haileyschoelkopf]

As requested by @LSinev ,

it might be helpful for some users to be able to run lm_eval or lm_eval/__main__.py without editing the file, but while using an LM subclass one has defined outside the library. This is already possible via either adding an import to __main__.py of the module containing the register_model() decorated class, or via passing an already-initialized LM class to simple_evaluate(), but that isn't as convenient in all cases.

We could thus allow, similar to the use of external task folders using --include_path, a way to import a desired python module at runtime, enabling users to import models / metrics defined outside the library into its registries.

[LSinev]

Are there any pitfalls to avoid (based on recent experience with merging PRs with new models)? Or anything to remember? In case of starting the PR development to close this issue?