Rename model_gen_kwargs -> generator_gen_kwargs

[saitcakmak]

Summary:
As titled. JSON storage is updated to support loading arguments serialized with the old name. There is no direct SQA storage for these fields (serialized through JSONified GeneratorSpec / GenerationStep)

Backwards compatibility for GStep & GSpec inputs are added later in the stack to ensure that this diff updates all internal usage.

Differential Revision: D89299127

[meta-codesync]

@saitcakmak has exported this pull request. If you are a Meta employee, you can view the originating Diff in D89299127.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.52%. Comparing base (b72fdf9) to head (229e00c).

Files with missing lines	Patch %	Lines
ax/generation_strategy/generation_node.py	87.50%	1 Missing ⚠️
...ion_strategy/generation_node_input_constructors.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4667   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files         568      568           
  Lines       58960    58966    +6     
=======================================
+ Hits        56910    56917    +7     
+ Misses       2050     2049    -1     

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[meta-codesync]

This pull request has been merged in 3d2ad4a.