Handle empty initializer axis scaling

[Akshay Sonawane (apsonawane)]

This pull request improves the robustness and correctness of the scale_by_axis method in the Initializer class and adds new tests to ensure proper handling of edge cases, especially with empty tensors and invalid scaler inputs.

Improvements to scale_by_axis implementation:

• Updated the calculation of block_size and num_blocks in the scale_by_axis method of the Initializer class to use more precise and consistent dimension handling, addressing potential bugs with axis indexing. (onnxruntime/core/optimizer/initializer.cc)

Testing enhancements:

• Added a new test case (ScaleByAxisEmptyTensor) to cover scenarios where the target or scaler tensors are empty, ensuring that the method does not throw unexpectedly and returns the correct size. The test also checks for proper exception handling with invalid scaler sizes and data types. (onnxruntime/test/optimizer/initializer_test.cc)
• Included the span_utils.h header in the test file to support the new test cases using spans. (onnxruntime/test/optimizer/initializer_test.cc)

[Copilot]

Pull request overview

Improves Initializer::scale_by_axis handling for empty tensors by avoiding division by zero.

Changes:

• Computes block counts directly from tensor dimensions.
• Adds empty-tensor and invalid-scaler tests.

Reviewed changes

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

File	Description
onnxruntime/core/optimizer/initializer.cc	Uses dimension products for scaling layout.
onnxruntime/test/optimizer/initializer_test.cc	Tests empty tensors and invalid scalers.

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