Add LLaMA C++ models and Fix PyType warning/errors raised in VS Code

[D-X-Y]

• Add a LLaMA C++ model at langfun/core/llms/llama_cpp.py
• Fix various minor pytype or annotation issues.

[daiyip]

Thanks for adding LLamaCpp models, Xuanyi! I have put a few comments inline.

[codecov-commenter]

Codecov Report

Merging #31 (f909534) into main (1ed40e7) will decrease coverage by 0.08%.
Report is 1 commits behind head on main.
The diff coverage is 91.89%.

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
- Coverage   97.27%   97.20%   -0.08%     
==========================================
  Files          38       39       +1     
  Lines        2860     2894      +34     
  Branches      597      602       +5     
==========================================
+ Hits         2782     2813      +31     
- Misses         31       33       +2     
- Partials       47       48       +1     

Files	Coverage Δ
langfun/core/llms/__init__.py	100.00% <100.00%> (ø)
langfun/core/llms/openai.py	94.89% <100.00%> (+0.21%)	⬆️
langfun/core/message.py	100.00% <ø> (ø)
langfun/core/llms/llama_cpp.py	89.65% <89.65%> (ø)