Move all step-based GS functionality to nodes (#4731)

[lena-kashtelyan]

Summary:

A lot going on here:

1. Make GenerationStep a factory for GenerationNode by replacing its __init__(self, ...) constructor (which would have to return a GStep) with a __new__(cls, ...) construtor, which can return a GNode (magic, thanks Devmate!)
2. Adapt storage: stop storing steps and just treat them as nodes. No step-only fields will be saved going forward. Backward compatibility is handled though.
3. Change a bazillion tests and checks in downstream applications.

NOTE: need to remember next steps here: https://www.internalfb.com/diff/D86066476?dst_version_fbid=732904879835890&transaction_fbid=1400151778204976, cc mgarrard

sorrybigdiff

Reviewed By: mgarrard

Differential Revision: D80128678

[meta-codesync]

@lena-kashtelyan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D80128678.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.05353% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.71%. Comparing base (7bce65b) to head (ad820b9).

Files with missing lines	Patch %	Lines
ax/generation_strategy/generation_node.py	95.12%	2 Missing ⚠️
ax/storage/sqa_store/decoder.py	81.81%	2 Missing ⚠️
ax/generation_strategy/generation_strategy.py	97.50%	1 Missing ⚠️
ax/storage/json_store/decoder.py	66.66%	1 Missing ⚠️
ax/storage/json_store/encoders.py	50.00%	1 Missing ⚠️
ax/utils/testing/modeling_stubs.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4746      +/-   ##
==========================================
- Coverage   96.72%   96.71%   -0.01%     
==========================================
  Files         583      583              
  Lines       60860    60807      -53     
==========================================
- Hits        58866    58811      -55     
- Misses       1994     1996       +2     

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