Create initializers not constant nodes in constant folding pass

[titaiwangms]

Partially From #2598

This provides a better optimized graph after constant folding in terms of the number of nodes, which is better for debugging.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.50%. Comparing base (8a94ad6) to head (cd4ea2b).

Files with missing lines	Patch %	Lines
onnxscript/optimizer/_constant_folding.py	80.00%	1 Missing and 2 partials ⚠️

❗ There is a different number of reports uploaded between BASE (8a94ad6) and HEAD (cd4ea2b). Click for more details.

HEAD has 21 uploads less than BASE

Flag	BASE (8a94ad6)	HEAD (cd4ea2b)
	42	21

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2650      +/-   ##
==========================================
- Coverage   70.46%   62.50%   -7.96%     
==========================================
  Files         224      224              
  Lines       26572    26573       +1     
  Branches     2637     2637              
==========================================
- Hits        18723    16609    -2114     
- Misses       6928     9184    +2256     
+ Partials      921      780     -141     

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