Added support for additional pythia-160m seeds

[will-hath]

Description

Added support for loading in models pythia-160m-seed1, pythia-160m-seed2, pythia-160m-seed3 with HookedTransformers, as well as a test to confirm that all official model names exist. Added specs to documentation as well.

Type of change

Please delete options that are not relevant.

• support for new pythia seeds

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

[jbloomAus]

@will-hath model_properties_table.md is generated automatically, did you manually edit it and is this why we have a merge conflict? It's possible this isn't the case in which case possible you just need to rebase to solve it.

Other than merge conflict, seems good. Thanks!