LLM Provider & Message Abstractions

[saitcakmak]

Summary: This diff implements a simple LLMProvider protocol (to be followed with a LiteLLMProvider implementation for easy access to many providers) and an LLMMessage dataclass. These classes support easy typing for any LLM usage within Ax, and enable storage of the inputs and conversations.

Differential Revision: D90788634

[meta-codesync]

@saitcakmak has exported this pull request. If you are a Meta employee, you can view the originating Diff in D90788634.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.71%. Comparing base (1eb22a8) to head (e0f11fc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4826   +/-   ##
=======================================
  Coverage   96.71%   96.71%           
=======================================
  Files         586      588    +2     
  Lines       61323    61366   +43     
=======================================
+ Hits        59309    59352   +43     
  Misses       2014     2014           

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