Fix TensorPrimitives.IndexOfMax

[lilinus]

Fixes #124233
Fixes #127610

Tried to include feedback from #124274 (comment).

Summary of changes:

• Change interface IIndexOfOperator to specialized IIndexOfMinMaxOperator, where there are Compare methods that returns masks of results/indices.
• IndexOfMinMaxCore delegates to ten different methods:
  • IndexOfMinMaxVectorized128/256/512Size4Plus when sizeof(T) is 4 or 8. The result index fits in one vector.
  • IndexOfMinMaxVectorized128/256/512Size2 when sizeof(T) is 2. The result index fits in two vectors.
  • IndexOfMinMaxVectorized128/256/512Size1 when sizeof(T) is 1. The result index fits in four vectors.
  • IndexOfMinMaxFallback as fallback.
• For vector methods: the final aggregation is done by horizontal-aggregation values in the lanes. Then the corresponding index found by matching that value bitwise.
• The search is done left-to-right so there is no need for the IndexLessThan methods
• Reintroduces some commented out TODO unit tests related to IndexOf methods

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR refactors the IndexOfMin/Max* tensor primitives to fix incorrect indices (notably for small element sizes) by introducing a specialized min/max operator interface and a new IndexOfMinMaxCore implementation with multiple vectorized paths.

Changes:

• Replaced IIndexOfOperator with IIndexOfMinMaxOperator and moved/rewrote IndexOfMinMaxCore into shared code.
• Implemented specialized Vector128/256/512 routines for sizeof(T) = 1/2/4/8 plus a naive fallback.
• Added regression tests for IndexOfMax on byte/ushort when the correct index exceeds the element type’s max value.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/libraries/System.Numerics.Tensors/tests/TensorPrimitives.Generic.cs	Adds regression tests for IndexOfMax returning indices > 255 and > 65535.
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorPrimitives.IndexOfMinMagnitude.cs	Updates operator to the new interface and comparison/aggregation model.
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorPrimitives.IndexOfMin.cs	Updates operator to the new interface and comparison/aggregation model.
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorPrimitives.IndexOfMaxMagnitude.cs	Updates operator to the new interface and comparison/aggregation model.
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorPrimitives.IndexOfMax.cs	Replaces per-method core logic with the shared core + new operator shape.
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/Common/TensorPrimitives.IIndexOfOperator.cs	Introduces IIndexOfMinMaxOperator and the new shared vectorized implementations.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

[tannergooding]

/ba-g networking test timeout