Unify BuilderBase and GraphBuilder: shared call_op with feature flags

[gramalingam]

Summary

Extends the BuilderBase / TapeBuilder infrastructure (from PR #2905) so that GraphBuilder inherits from BuilderBase, sharing a single call_op() implementation gated by feature flags.

Changes

Class rename and API merge

• Renamed OpBuilderBase → BuilderBase (no backward-compat alias needed — not yet released)
• Merged the separate op() and _make_node() methods into a single op(op_type, *args, **kwargs) ergonomic API
• Promoted _create_op to BuilderBase.call_op() — the single core node-creation method

Feature flags (BuilderFeature enum)

• SCHEMA_PARTITION — partition positional args into inputs vs attributes using op schema
• CAST_INPUTS — promote scalars to tensors, match type variables
• CAST_ATTRIBUTES — cast attributes using schema info (stub for future)
• INFER_SHAPES — run shape inference after node creation
• CONSTANT_PROPAGATION — run constant propagation after node creation
• FULL = all of the above (used by GraphBuilder)
• NONE = no extra processing (used by TapeBuilder for rewriter/optimizer/version-converter)

GraphBuilder inherits from BuilderBase

• GraphBuilder(BuilderBase) with features=BuilderFeature.FULL
• call_op() is inherited directly — no override needed
• Default hook implementations (_get_schema, _partition_inputs_attributes, _cast_inputs, _cast_attributes) live in BuilderBase
• GraphBuilder only overrides hooks needing graph-specific state:
  • _input_to_ir_value — constant promotion with caching
  • _generate_node_name / _adapt_outputs — qualified naming
  • _post_create_node — constant propagation + shape inference
  • _annotate_node — scope metadata

Exports

• BuilderFeature exported from onnxscript package

Testing

• All 559 rewriter/optimizer/version-converter tests pass
• All 90 GraphBuilder tests pass
• All 15 TapeBuilder/context tests pass

[codecov]

Codecov Report

❌ Patch coverage is 88.54167% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.63%. Comparing base (029441f) to head (d3b52dd).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/_internal/tape_builder.py	86.80%	14 Missing and 5 partials ⚠️
onnxscript/_internal/builder.py	91.89%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2910      +/-   ##
==========================================
+ Coverage   72.61%   72.63%   +0.01%     
==========================================
  Files         259      259              
  Lines       31597    31651      +54     
  Branches     2973     2980       +7     
==========================================
+ Hits        22945    22990      +45     
- Misses       7643     7653      +10     
+ Partials     1009     1008       -1     

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