Validate empty reduction axes

[Akshay Sonawane (apsonawane)]

This pull request improves the handling and validation of empty set reductions in the ONNX Runtime CPU reduction operators. It enforces stricter checks for the axes input, prevents unnecessary memory operations, and adds comprehensive tests to verify correct behavior for various edge cases.

Validation and Error Handling Improvements:

• Enforces that the axes tensor input must be a 1D vector and only processes it if it is present, improving robustness and error messages for invalid input shapes.
• Adds a check to ensure that memory copying only occurs when the input tensor is non-empty, avoiding redundant operations.

Logic and Flow Adjustments:

• Refactors the order of reduction logic to first handle empty axes cases before checking for empty set input, ensuring correct execution flow and output. [1] [2]

Testing Enhancements:

• Adds new tests to verify:
  • Reduction when the optional axes input is missing (should reduce all dimensions).
  • Validation that the axes tensor must be a vector, with expected error messages for invalid cases.
  • Correct behavior for the noop_with_empty_axes attribute, both when axes are omitted and when an empty axes tensor is provided.

[Copilot]

Pull request overview

Improves CPU reduction handling for empty tensors and optional axes.

Changes:

• Validates axes tensors as one-dimensional.
• Handles omitted or empty axes before empty-set reduction.
• Adds edge-case tests and avoids zero-byte copies.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
onnxruntime/core/providers/cpu/reduction/reduction_ops.cc	Updates empty-set validation and reduction flow.
onnxruntime/test/providers/cpu/reduction/reduction_ops_test.cc	Adds empty-axes and optional-axes tests.

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