Cleanup SymbolValue

[gramalingam]

More cleanup (in the migration of converter to onnx IR):

• Eliminate some redundant info/logic (class Dynamic)
• Cleanup AttrRef and SymbolValue.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.09%. Comparing base (8c6084c) to head (c8c2f57).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/_internal/converter.py	81.08%	2 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2752      +/-   ##
==========================================
- Coverage   70.10%   70.09%   -0.01%     
==========================================
  Files         228      228              
  Lines       27393    27382      -11     
  Branches     2781     2783       +2     
==========================================
- Hits        19203    19194       -9     
+ Misses       7234     7229       -5     
- Partials      956      959       +3     

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