Tensor<T> select. Iteration along dimensions

[michaelgsharp]

Fixes #113068 and fixes #113863

Adds a new method, SliceAlongDimension, that lets you slice any given subdimension out of a tensor.

Adds a new method, Select, that lets you run Tensor operations on segments of the Tensor instead of the whole Tensor.

Adds a DimensionCollection, which is similar to a Dictionary's KeyCollection, in that it has an Enumerator that lets you iterate over the Tensor along a given dimension (compared to the KeyCollection that lets you iterate by the keys).

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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[tjwald]

My comments on the new API:
#113068 (comment)

[ericstj]

@michaelgsharp what's the state of this PR? Are you returning to it, or is it no longer necessary?

[Copilot]

Pull Request Overview

This PR introduces new slicing and selection operations for Tensor by adding methods such as SliceAlongDimension and Select, as well as new types for iterating along tensor dimensions. Key changes include new unit tests for dimension slicing, additions to the Tensor and TensorSpan implementations to support GetDimensionSpan, and corresponding project file and suppression updates.

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/TensorDimensionSpanTests.cs	Added thorough tests for dimension slicing on tensors in various shapes
src/System/Numerics/Tensors/netcore/Tensor_1.cs	New GetDimensionSpan method and tweak in IDisposable.Dispose implementation
src/System/Numerics/Tensors/netcore/TensorSpan_1.cs	Added GetDimensionSpan method for TensorSpan
src/System/Numerics/Tensors/netcore/TensorShape.cs	Added AdjustToNextIndex and updated GetLinearOffset methods using DivRem
src/System/Numerics/Tensors/netcore/TensorDimensionSpan_1.cs	New implementation of TensorDimensionSpan for slicing a tensor along a given dimension
csproj and CompatibilitySuppressions.xml	Updated to include the new files and suppress expected compatibility diagnostics

Comments suppressed due to low confidence (1)

src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorDimensionSpan_1.cs:25

• Consider adding a clarifying comment here to explain why the 'dimension' parameter is incremented, which would help improve readability and understanding of the slicing logic.

dimension += 1;

[ericstj]

Looks good for the most part - shared a suggestion to cleanup suppressions and bring ref up to date.

[ericstj]

Should this implement other interfaces in older TFMs?

[tannergooding]

What other interfaces would it implement?

[tannergooding]

/ba-g unrelated failures