Add parent/root tracking to GraphBuilder for subgraph Parameter realization

[gramalingam]

GraphBuilder now accepts an optional parent parameter. When provided, _root points to the root builder of the parent chain, ensuring that Parameter._realize() registers initializers in the main (root) graph rather than in the subgraph where the parameter is referenced.

This is needed for ONNX Scan/Loop body subgraphs where nn.Module parameters must be stored in the root graph instead of the attribute subgraph.

Changes:

• GraphBuilder.init: accept parent, store _parent and _root
• build_graph(): accept and forward parent to sub-builder
• GraphBuilder.subgraph(): pass parent=self to build_graph()
• Parameter._realize(): use builder._root for name qualification and initializer registration

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.03%. Comparing base (1077da7) to head (e20440c).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2877      +/-   ##
==========================================
+ Coverage   71.96%   72.03%   +0.06%     
==========================================
  Files         239      239              
  Lines       29224    29295      +71     
  Branches     2878     2879       +1     
==========================================
+ Hits        21031    21102      +71     
  Misses       7216     7216              
  Partials      977      977              

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