Clean up center generation node implementation

[mgarrard]

Summary:
This diff cleans up center generation node logic to hopefully make it a bit more eloquent. It does the following:

• Removes AutoTransitionAfterGenOrExhaustion as it is duplicative of AutoTransitionAfterGen, it just has a different defalt
• Updates base ExternalGenerationNode to have a gen override which allows for should_skip to be set by a helper method, and called prior to super.gen() -- this allows for skiping to for special cases like center_generation_node, without updating the core logic in generation strategy (which would have required checking after fitting, and that would have determential consequences on generation time)
• stores center parms in update_generator_state - prevents any recomputations that are unnecessary, center will remain the same for one given experiment state at time of gen
• removes custom gen override in center_generation_node

Notably, it would be only in a misconfigured state that we would use sobol fallback now, this is consistent with David's initial changes, see D87797277 for explaination of that chage -- we could even remove sobol fallback from the node if we want

Differential Revision:
D90040731

Privacy Context Container: L1307644

[meta-codesync]

@mgarrard has exported this pull request. If you are a Meta employee, you can view the originating Diff in D90040731.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.32353% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.72%. Comparing base (60f5ca9) to head (0d5e25a).

Files with missing lines	Patch %	Lines
ax/core/search_space.py	93.84%	4 Missing ⚠️
ax/generation_strategy/center_generation_node.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4734      +/-   ##
==========================================
- Coverage   96.72%   96.72%   -0.01%     
==========================================
  Files         582      582              
  Lines       60699    60723      +24     
==========================================
+ Hits        58713    58736      +23     
- Misses       1986     1987       +1     

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