Create _testing/models sub-folder and move llama3 there (#144)

* Create _testing/models sub-folder and move llama3 there

I'm going to be starting adding tess specific to llama3, so having the model easily accessible without copying it would be good. Plus, with the pipeline example in #121 we already have the implementation of llama3 which is duplicated, so let's keep it in a sub-folder

* py39 typing

Author: fmassa