[Akshay Sonawane (apsonawane)]

This pull request updates the logic for determining whether a node's input is a valid initializer for fusion and adds a new unit test to ensure that initializers which can be overridden (i.e., initializers that are also graph inputs) are properly skipped during fusion. The main changes are:

Logic Update

• In IsNodeValidForFusion, the check for a valid initializer now uses graph_utils::GetConstantInitializer, ensuring that only constant initializers (not those that can be overridden by graph inputs) are considered valid for fusion.

Testing Improvements

• Added a new test, FuseInitializersSkipsOverridableInitializer, to verify that the transformer skips fusion for initializers that are also graph inputs and can be overridden. This test ensures the fusion transformation does not incorrectly modify such initializers.
• Included the graph_transform_test_builder.h header to support the new test.

[Copilot]

Pull request overview

Prevents fusion of overridable initializers, preserving runtime input semantics.

Changes:

• Uses constant-initializer detection before fusion.
• Adds regression coverage for initializer-backed graph inputs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
onnxruntime/core/optimizer/fuse_initializers_transformer.cc	Restricts fusion to constant initializers.
onnxruntime/test/optimizer/fuse_initializers_transformer_test.cc	Tests that overridable initializers remain unfused.

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